Courseiva

CCNA OSPF Questions

72 of 147 questions · Page 2/2 · OSPF topic · Answers revealed

76
Drag & Dropmedium

Drag and drop the following OSPFv2 neighbor state transitions into the correct order, starting from the initial Down state on a broadcast or point-to-point network (non-NBMA).

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4

Why this order

The standard OSPF neighbor state machine on broadcast and point-to-point networks proceeds: Down, Init, 2-Way, ExStart, Exchange, Loading, Full. The Attempt state exists only on NBMA networks and is not used here. Option D correctly lists the first five states in order: Down → Init → 2-Way → ExStart → Exchange.

Other options incorrectly include the NBMA-only Attempt state or misorder the states like Loading before Exchange.

Exam trap

Do not confuse the standard OSPF neighbor states with the NBMA-specific Attempt state; unless the network type is explicitly NBMA, assume the common broadcast/point-to-point order omitting Attempt.

77
Drag & Dropmedium

Drag and drop the following steps into the correct order to configure and verify OSPFv3 neighbor adjacency using link-local addresses in area 0.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

The correct order enables IPv6 unicast routing globally, creates the OSPFv3 process, enters interface configuration, assigns the interface to the OSPFv3 process in area 0 (which activates OSPFv3 on the link), and finally verifies the neighbor relationship. IPv6 unicast routing must be enabled first because OSPFv3 requires IPv6 packet forwarding. The router process must exist before an interface can be associated with it.

Assigning the interface to area 0 triggers Hello packet exchange over link-local addresses. The show command confirms the adjacency formed.

78
MCQhard

Refer to the exhibit. A network administrator is troubleshooting why not all OSPF neighbors are fully adjacent on a multi-access broadcast segment. After issuing the show ip ospf neighbor command on R1, the output is displayed. What is the most likely cause of the 2WAY/DROTHER state for neighbor 172.16.1.1?

A.The neighbor is a DROther and a full adjacency with another DROther is not required; adjacency is formed only with the DR and BDR.
B.The OSPF hello and dead timers are mismatched between R1 and neighbor 172.16.1.1.
C.The network type is configured as point-to-point on one side and broadcast on the other, causing a DR/BDR election failure.
D.The neighbor's router ID 172.16.1.1 is not reachable, preventing the completion of the adjacency process.
AnswerA

The exhibit shows neighbor 172.16.1.1 in state 2WAY/DROTHER. In a broadcast multi-access OSPF network, DROthers exchange hellos and reach 2-Way state with each other but stop at that stage, establishing full adjacency only with the DR and BDR. This is standard OSPF behavior and the most likely reason for the state.

Why this answer

In a multi-access broadcast OSPF network, only the Designated Router (DR) and Backup Designated Router (BDR) form full adjacencies with all other routers. DROthers (routers that are neither DR nor BDR) only form full adjacencies with the DR and BDR, and remain in the 2WAY state with other DROthers. The show ip ospf neighbor output on R1 shows neighbor 172.16.1.1 as 2WAY/DROTHER, which is normal behavior because both are DROthers and do not need to exchange LSAs directly.

Exam trap

Cisco often tests the misconception that any neighbor state other than FULL is a problem, but here the trap is that 2WAY/DROTHER is actually expected behavior for non-DR/BDR routers on a broadcast segment, and candidates may incorrectly assume a timer mismatch or reachability issue.

Why the other options are wrong

B

Candidates mistakenly think any non-FULL state implies a timer mismatch, overlooking that timer issues prevent even reaching 2WAY.

C

Some candidates assume any DR/BDR-related issue indicates a network type mismatch, but the exhibit clearly shows successful DR/BDR formation, ruling out this option.

D

Candidates may confuse LSA reachability with neighbor adjacency requirements. The 2WAY state proves basic connectivity is intact.

79
MCQhard

A network engineer configures a static route: ip route 192.168.10.0 255.255.255.0 10.1.1.2. The next hop 10.1.1.2 is reachable via OSPF. Later, the engineer notices that the route to 192.168.10.0/24 has disappeared from the routing table. What is the most likely cause?

A.The OSPF route to 10.1.1.0/30 has been lost, making the next-hop address 10.1.1.2 unresolvable.
B.The static route has a higher administrative distance than OSPF, so OSPF's route to 192.168.10.0/24 replaced it.
C.The static route uses a next-hop IP address that is not directly connected, which is unsupported on this platform.
D.A routing loop caused by recursive lookups has suppressed the static route to prevent loops.
AnswerA

A recursive static route requires a next-hop that is resolvable via an existing route. Without the OSPF route to the subnet containing 10.1.1.2, the router cannot reach the next hop and removes the static route from the routing table.

Why this answer

The static route to 192.168.10.0/24 uses 10.1.1.2 as the next-hop address. For a static route with a next-hop IP (rather than an exit interface) to be installed in the routing table, the next-hop must be reachable via a valid route in the routing table. If the OSPF route to the subnet containing 10.1.1.2 (e.g., 10.1.1.0/30) is lost, the next-hop becomes unreachable, and the static route is removed from the routing table.

This is the most likely cause of the route disappearing.

Exam trap

Cisco often tests the concept that a static route with a next-hop IP requires a valid route to that IP in the routing table, and candidates mistakenly think the static route will always remain or that administrative distance is the cause of its removal.

Why the other options are wrong

B

Believing that a static route is removed from the routing table when a better route exists, rather than understanding it remains but is inactive.

C

Assuming that a static route's next-hop must be directly attached, ignoring that recursive routing via another routing protocol or static route is allowed.

D

Thinking that recursive static routes are prone to loop suppression, rather than understanding that the route is simply withdrawn when the next-hop is no longer reachable.

80
MCQmedium

A router advertises its LAN network into OSPF, but no OSPF Hellos should be sent toward end-user devices on that LAN. Which configuration approach solves this cleanly?

A.Use ip ospf cost 65535 on the LAN interface
B.Configure the LAN interface as a passive interface in OSPF
C.Disable OSPF globally and redistribute connected routes
D.Convert the LAN interface to a loopback
AnswerB

The `passive-interface` command under the OSPF process tells OSPF to stop transmitting Hello packets on that interface while still injecting the interface's subnet as an OSPF route. This preserves full route reachability into the LAN without forming neighbor adjacencies, cutting unnecessary protocol overhead. It is the standard, purpose-built mechanism for LAN-facing OSPF interfaces.

Why this answer

A passive interface advertises the connected subnet into OSPF without sending or processing Hellos on that interface. That is exactly what you want for user-facing LANs with no OSPF neighbors.

Exam trap

A frequent exam trap is selecting the option to increase the OSPF cost on the LAN interface to prevent Hello packets. While adjusting the cost changes the metric used for route selection, it does not stop the router from sending or receiving OSPF Hellos. Another common mistake is to disable OSPF globally and redistribute connected routes, which is unnecessarily complex and does not address the problem of suppressing Hellos on a specific interface.

Additionally, converting the LAN interface to a loopback is irrelevant because loopbacks are logical interfaces used for router IDs and testing, not for controlling OSPF Hello behavior on physical LAN interfaces.

Why the other options are wrong

A

Using 'ip ospf cost 65535' on the LAN interface only changes the OSPF metric for that interface. It does not prevent the router from sending or receiving OSPF Hello packets, so neighbor adjacencies can still form, which is not the desired behavior.

C

Disabling OSPF globally and redistributing connected routes is an overly complex and unnecessary approach. It does not selectively stop Hello packets on the LAN interface and can introduce routing complexity and instability.

D

Converting the LAN interface to a loopback interface changes the interface type entirely and is not a standard or practical solution for suppressing OSPF Hellos on a LAN. Loopbacks are logical interfaces used for router IDs and testing, not for controlling OSPF Hello behavior.

81
MCQmedium

Which OSPF network type on Ethernet performs a DR and BDR election by default?

A.Point-to-point
B.Broadcast
C.Point-to-multipoint nonbroadcast
D.Loopback
AnswerB

On Ethernet segments, OSPF automatically defaults to the broadcast network type. This type relies on multicast hello packets to 224.0.0.5 and elects a Designated Router (DR) and Backup Designated Router (BDR) to minimize the number of adjacencies and reduce LSA flooding on the multi-access link. Therefore, broadcast is the correct network type where DR/BDR election occurs.

Why this answer

Broadcast multiaccess networks such as Ethernet elect a DR and BDR by default.

Exam trap

A frequent exam trap is selecting point-to-point or point-to-multipoint nonbroadcast as the network type that performs DR/BDR elections. Candidates often assume any multi-router link requires DR/BDR, but OSPF only elects DR/BDR on broadcast and NBMA networks. Point-to-point links connect exactly two routers and do not need DR/BDR, while point-to-multipoint nonbroadcast requires manual neighbor configuration and does not elect DR/BDR by default.

Misunderstanding these distinctions leads to incorrect answers. Remember, Ethernet interfaces default to broadcast network type, which triggers DR/BDR elections automatically.

Why the other options are wrong

A

Point-to-point links connect only two routers directly and do not require or perform DR/BDR elections because there is no need to reduce flooding on a single link. Selecting this option is incorrect for Ethernet interfaces that default to broadcast.

C

Point-to-multipoint nonbroadcast network type requires manual neighbor configuration and does not perform DR/BDR elections by default. This does not match the default behavior of Ethernet interfaces.

D

Loopback interfaces are virtual interfaces used for router identification and do not participate in OSPF DR/BDR elections because they are not multiaccess networks.

82
MCQhard

A router has an OSPF-learned route to a destination prefix and also a directly connected route to a broader supernet that includes that destination. The OSPF route is more specific. Which route is used for the destination?

A.The more specific OSPF route
B.The directly connected broader route
C.Both routes equally
D.Neither route can be used because the sources differ
AnswerA

This is correct because the most specific matching prefix is preferred first.

Why this answer

The more specific OSPF route is used. In practical terms, route specificity is checked before broader route-source considerations when the prefixes are different. Even though the connected route is a directly attached source and often strongly trusted, it still loses if it is less specific than another matching route.

This is a subtle route-selection question because it combines source type and specificity. The key is that longest-prefix match comes first.

Exam trap

A common exam trap is assuming that directly connected routes always take precedence over OSPF routes because they have a lower administrative distance. Candidates may incorrectly select the broader directly connected route, forgetting that routers first apply longest prefix match before considering administrative distance. This leads to the mistaken belief that a less specific connected route overrides a more specific OSPF route.

The trap is confusing route source preference with prefix specificity, which can cause incorrect answers on routing questions involving multiple route sources.

Why the other options are wrong

B

This option is incorrect because a directly connected route, even though it has a lower administrative distance, does not override a more specific OSPF route due to prefix length precedence.

C

This option is incorrect because routers do not use multiple routes equally when one route is a more specific match; only the best matching route is used for forwarding.

D

This option is incorrect because routers can compare and select routes from different sources; differing sources do not prevent route usage if one route is the best match.

83
PBQhard

You have console access to both R1 and R2. Configure OSPFv2 on both routers to establish a single-area adjacency in area 0. The link between R1 and R2 uses 10.0.0.0/30. Currently, OSPF is not configured on either router. After configuration, verify the adjacency forms and routes are exchanged.

Network Topology
G0/010.0.0.1/30G0/010.0.0.2/3010.0.0.0/30R1R2

Hints

  • Use 'router ospf <process-id>' to enter OSPF configuration mode.
  • The network statement uses a wildcard mask, not a subnet mask. For a /30, use 0.0.0.3.
  • Remember to set a router-id; it can be any IP address, but must be unique per router.
A.Configure 'router ospf 1' on R1 and R2, set router-id, and use 'network 10.0.0.0 0.0.0.3 area 0' on both routers.
B.Configure 'router ospf 1' on R1 only, and use 'network 10.0.0.0 0.0.0.3 area 0' on R1; R2 does not need OSPF configuration because it will learn routes via the directly connected interface.
C.Configure 'router ospf 1' on both routers, set router-id, and use 'network 10.0.0.0 255.255.255.252 area 0' on both routers.
D.Configure 'router ospf 1' on both routers, set router-id, and use 'network 10.0.0.0 0.0.0.0 area 0' on both routers.
AnswerA
solution
! R1
configure terminal
router ospf 1
router-id 1.1.1.1
network 10.0.0.0 0.0.0.3 area 0
network 192.168.1.0 0.0.0.255 area 0
network 10.1.1.1 0.0.0.0 area 0
passive-interface GigabitEthernet0/1
passive-interface Loopback0
end

Why this answer

The essential requirement is enabling OSPFv2 on both routers with matching area 0 on the 10.0.0.0/30 link. On R1 and R2, enter 'router ospf 1', set a unique router-id, and use 'network 10.0.0.0 0.0.0.3 area 0' to advertise the link. The solution commands include optional networks (192.168.1.0/24 and Loopback0) that are not required for the adjacency and are shown only as examples; candidates should focus on the link network.

After configuration, 'show ip ospf neighbor' should show a FULL state. Common mistakes include using a subnet mask instead of a wildcard mask (option C) or a /32 wildcard (option D), and not configuring OSPF on R2 (option B).

Exam trap

Remember that OSPF network statements use wildcard masks, not subnet masks. Also, both routers must be configured; OSPF does not automatically enable on all interfaces. Use 'show ip ospf neighbor' to verify adjacency formation.

Why the other options are wrong

B

The specific factual error is that OSPF is a dynamic routing protocol that must be enabled on both ends of a link for adjacency to form; one-sided configuration does not work.

C

The specific factual error is confusing subnet masks with wildcard masks; OSPF uses inverse masks in network statements.

D

The specific factual error is using a host wildcard mask that does not cover the actual interface IPs; the correct wildcard mask must include the range of IPs on the link.

84
MCQhard

R1 and R2 are directly connected and both configured for OSPF area 0. The IP addresses are correct, but the routers do not become neighbors. What is the most likely cause?

A.The OSPF network types on the interfaces do not match.
B.The routers need matching hostnames to exchange LSAs.
C.The /30 subnet is too small for OSPF to operate.
D.The interfaces must be converted into switch trunks.
AnswerA

This is correct because OSPF network type controls fundamental adjacency behaviors such as Hello/Dead timer intervals and DR/BDR election. A point-to-point interface expects to form an adjacency without a DR, while a broadcast interface will wait for a DR/BDR process, and their timer values typically differ. Even if both routers have matching subnet masks and are directly connected, a mismatch in network type causes the routers to discard each other's Hello packets or fail to reach Full state, preventing any LSA exchange.

Why this answer

The most likely cause is an OSPF network type mismatch. In practical terms, both routers are on the same IP segment and both are trying to use OSPF in the same area, but they do not agree on the type of OSPF network the interface represents. That matters because OSPF behavior changes depending on the network type, including how neighbors are discovered and how adjacencies are formed.

This is a classic exam-style troubleshooting case because the obvious items look correct: IP addressing works and the area matches. But a mismatch between point-to-point and broadcast expectations can still stop the relationship from forming cleanly. That makes network type mismatch the strongest answer here.

Exam trap

A frequent exam trap is to overlook the importance of matching OSPF network types on connected interfaces. Candidates might assume that correct IP addressing and area numbers are sufficient for adjacency. However, if one router uses a broadcast network type and the other uses point-to-point, they will not become neighbors despite appearing correctly configured.

This subtle mismatch is often missed because it does not generate explicit errors, leading to confusion and incorrect troubleshooting steps.

Why the other options are wrong

B

This option is incorrect because OSPF neighbor relationships do not depend on matching hostnames. Hostnames are administrative identifiers and do not affect OSPF protocol operations or LSA exchanges.

C

This option is incorrect because a /30 subnet is a standard subnet size for point-to-point links and does not prevent OSPF from operating or forming adjacencies. OSPF works normally over /30 subnets.

D

This option is incorrect because OSPF runs over routed interfaces and does not require interfaces to be configured as switch trunks. Trunking is related to VLAN tagging and switching, not OSPF adjacency.

85
PBQmedium

You are connected to R1 via console. R1 is connected to R2 via GigabitEthernet0/0 (10.0.0.1/30) and to R3 via GigabitEthernet0/1 (10.0.0.5/30). R1 has a management subnet 192.168.1.0/24 connected to GigabitEthernet0/2. The network administrator wants to ensure that traffic from the management subnet to the Internet (203.0.113.0/24) uses R2 as the primary path and R3 as a backup. Currently, OSPF is running with default metrics. You must configure a floating static route that will be used only if the OSPF route fails.

Network Topology
G0/010.0.0.1/30G0/110.0.0.5/30G0/2192.168.1.1/24R1R2R3Management subnet

Hints

  • Floating static route has a higher administrative distance than the dynamic route you want to back up.
  • The next-hop IP for the backup route is R3's interface IP connected to R1.
  • Check the OSPF administrative distance (default 110) and set the static route's AD accordingly.
A.ip route 203.0.113.0 255.255.255.0 10.0.0.6 150
B.ip route 203.0.113.0 255.255.255.0 10.0.0.2 150
C.ip route 203.0.113.0 255.255.255.0 10.0.0.6 110
D.ip route 0.0.0.0 0.0.0.0 10.0.0.6 150
AnswerA
solution
! R1
ip route 203.0.113.0 255.255.255.0 10.0.0.6 150

Why this answer

The floating static route is configured with an administrative distance of 150, which is higher than OSPF's default AD of 110. This ensures that the static route is only installed in the routing table when the OSPF route is not present (e.g., due to a failure). The next-hop is R3's IP on the directly connected link.

Exam trap

Remember that a floating static route requires an administrative distance higher than the dynamic protocol's default. Also, ensure the next-hop is the backup router's IP, not the primary. Do not confuse the destination network with a default route unless the question asks for Internet access in general.

Why the other options are wrong

B

The next-hop should be R3 (10.0.0.6), not R2 (10.0.0.2).

C

The AD must be higher than OSPF's 110 (e.g., 150) to act as a floating static route.

D

The destination network should be 203.0.113.0/24, not a default route.

86
MCQhard

Exhibit: OSPF neighbors are not reaching FULL state on an Ethernet segment with multiple routers. The output of show ip ospf neighbor on R2 shows a neighbor in the 2WAY/DROTHER state. What is the most likely reason?

A.Authentication mismatch between R2 and 3.3.3.3
B.The 2WAY state with another DROTHER on a broadcast segment is normal
C.R2 must be configured as a point-to-point network type
D.R2 has a duplicate router ID
AnswerB

On a broadcast multiaccess segment, OSPF elects a DR and BDR; all other routers are DROTHERs. DROTHERs only form full adjacencies with the DR and BDR, while two DROTHERs remain in 2WAY, which is the normal and expected state. The exhibit showing R2 in 2WAY with another DROTHER is therefore not a problem, so this is the correct explanation.

Why this answer

On a broadcast Ethernet network, two routers normally become fully adjacent through the DR or BDR. If the local router is stuck in 2WAY with another DROTHER, that is normal behavior. It is not a fault by itself.

Exam trap

A common exam trap is assuming that neighbors stuck in the 2-Way state indicate a problem requiring troubleshooting or configuration changes. Many candidates mistakenly believe that all OSPF neighbors on a broadcast segment must reach the FULL state with each other. However, OSPF’s design limits full adjacency to DR and BDR routers only.

DROTHER routers remain in 2-Way state with each other, which is normal and expected. Misinterpreting this behavior can lead to incorrect answers such as blaming authentication or router ID issues when the output actually reflects standard OSPF operation.

Why the other options are wrong

A

Authentication mismatches prevent OSPF neighbors from reaching the 2-Way state by blocking bidirectional communication. Since the neighbors here have reached 2-Way, authentication mismatch is unlikely the cause.

C

Configuring the network type as point-to-point is not required on Ethernet segments and would change the adjacency formation behavior rather than explain the current normal 2-Way state with DROTHER routers.

D

Duplicate router IDs cause adjacency failures and routing problems but do not cause neighbors to remain in the 2-Way state with DROTHER routers. The output does not indicate a router ID conflict.

87
MCQhard

An engineer configures a floating static route to 0.0.0.0/0 with an administrative distance of 200 while OSPF is providing a default route. What is the intended behavior?

A.The static default route acts as a backup and becomes active only if the OSPF default route is lost.
B.The static default route overrides OSPF immediately because it is manually configured.
C.Both default routes must always load-balance together.
D.The router ignores both defaults because they overlap.
AnswerA

A floating static route is configured with an administrative distance greater than OSPF's default of 110, such as 150. While the OSPF default route exists in the routing table, the static route remains dormant because the router prefers the lower AD. If the OSPF route disappears due to a neighbor loss or removal of the default-information originate command, the static route is then installed and used as the default path.

Why this answer

The intended behavior is that the static default route stays in reserve and becomes active only if the OSPF-learned default route disappears. In plain language, the administrator wants a backup path, not a replacement for the normal OSPF path. By assigning the static route a higher administrative distance than OSPF, the router treats it as less trustworthy during normal operation.

This is a standard floating-static design. The static route is still configured, but it does not normally appear as the preferred forwarding choice until the lower-distance route is lost. That is the key operational purpose of the configuration.

Exam trap

Don't assume static routes always take precedence over dynamic ones; administrative distance dictates preference.

Why the other options are wrong

B

The static default route has an administrative distance of 200, which is higher than OSPF's default distance of 110. Therefore, OSPF's route is preferred, and the static route does not override it. Manual configuration does not bypass administrative distance; the router always prefers the lower distance.

C

Load balancing requires multiple routes with equal administrative distance and metric. Here, OSPF and the static route have different administrative distances (110 vs. 200), so they are not equal. The router will only use the best route (OSPF) and not load-balance.

D

Overlapping default routes are common and do not cause the router to ignore them. The router uses the route with the lowest administrative distance (OSPF) and ignores the static route unless OSPF fails. There is no conflict that would cause both to be ignored.

88
MCQhard

A router learns the same destination from EIGRP and OSPF. The EIGRP route has a metric of 1000, and the OSPF route has a metric of 10. Which route is installed by default?

A.The OSPF route, because 10 is lower than 1000
B.The EIGRP route, because its source has a lower default administrative distance
C.Both routes automatically install for load balancing
D.Neither route installs until the administrator chooses manually
AnswerB

The EIGRP route is chosen because Cisco's default administrative distance for EIGRP is 90, while OSPF uses a default AD of 110. Since the router compares AD first to decide which protocol's route to trust, the lower AD of EIGRP makes it the preferred source for the destination, and OSPF's route is held as a backup only. The metric values, no matter how attractive, are not considered until the AD comparison is resolved, so EIGRP wins outright.

Why this answer

The EIGRP route is installed by default because route selection between different routing protocols is based on administrative distance before the protocol-specific metric is compared across sources. In plain language, the router does not compare an OSPF metric of 10 directly against an EIGRP metric of 1000 because those metrics come from different systems and are not numerically comparable in a meaningful cross-protocol way. Instead, the router first looks at the trustworthiness of the source.

By default, internal EIGRP routes have a lower administrative distance than OSPF routes, so EIGRP wins even though the OSPF metric value appears lower. This is a classic CCNA trap designed to catch people who compare metrics across different protocols without considering administrative distance first.

Exam trap

A common exam trap is to assume that the route with the numerically lowest metric is always preferred, regardless of the routing protocol. In this question, the OSPF route has a metric of 10, which looks better than the EIGRP metric of 1000. However, metrics from different protocols are not directly comparable.

The router first compares administrative distance, which rates the trustworthiness of the routing source. Since EIGRP’s default administrative distance (90) is lower than OSPF’s (110), the router installs the EIGRP route despite its higher metric. This trap tests your understanding of routing protocol preference, not just metric values.

Why the other options are wrong

A

This option is incorrect because it assumes the router compares OSPF and EIGRP metrics directly. Metrics from different protocols are not comparable until administrative distance is considered, so the lower OSPF metric does not guarantee route installation.

C

This option is incorrect because load balancing occurs only when multiple equal-cost routes exist within the same routing protocol. Routes from different protocols are not automatically load-balanced simply because they reach the same destination.

D

This option is incorrect because the router automatically selects the best route using its decision process based on administrative distance and metric. Manual intervention is not required for route installation in this scenario.

89
MCQhard

A router learns 172.16.0.0/16 from OSPF and 172.16.10.0/24 from a static route. Which route is used for traffic to 172.16.10.55?

A.The OSPF /16 route
B.The static /24 route
D.Neither route because the prefixes overlap
AnswerB

The static /24 route is correct because the destination 172.16.10.55 matches its prefix length of 24 bits (172.16.10.0/24), whereas the OSPF /16 route only matches the first 16 bits. Routers use longest prefix match (LPM) to select the most specific route, and the /24 is more specific than both the /16 and any default route. Even though OSPF may have a lower administrative distance, prefix length takes precedence over AD for route selection when prefixes differ, so the static /24 is installed in the forwarding table for this destination.

Why this answer

The static /24 route is used because it is more specific than the OSPF /16 route. In plain language, even though OSPF is a dynamic source and the /16 covers the destination broadly, the router prefers the entry that describes the exact destination range more precisely. Since 172.16.10.55 falls within 172.16.10.0/24, that route wins under longest-prefix match.

This is a classic example of route specificity taking priority before broader route-source comparisons would matter between equal prefix lengths.

Exam trap

A frequent exam trap is assuming that the dynamic OSPF route will always be preferred over a static route, regardless of prefix length. Many candidates overlook that the router prioritizes the longest-prefix match before considering administrative distance or route source. Because 172.16.10.0/24 is more specific than 172.16.0.0/16, the router uses the static route for traffic to 172.16.10.55.

Misunderstanding this can lead to incorrect answers, especially when both routes overlap. Remember, overlapping routes are common and resolved by prefix specificity, not by route type alone.

Why the other options are wrong

A

The OSPF /16 route is less specific than the static /24 route. Although OSPF is a dynamic routing protocol, the router prefers the route with the longer prefix length, so this option is incorrect.

C

The default route is only used when no specific matching route exists. Since both OSPF and static routes cover the destination, the default route is not used here, so this option is incorrect.

D

Overlapping prefixes are normal in routing tables and do not prevent route selection. The router resolves overlaps using longest-prefix match, so this option is incorrect.

90
Multi-Selectmedium

Which TWO statements correctly describe OSPFv2 router-id selection and verification in a single-area configuration?

Select 2 answers
A.The OSPF router-id is automatically derived from the MAC address of the first Ethernet interface.
B.If the router-id is changed using the 'router-id' command, the change takes effect immediately without any additional action.
C.The router-id must be the same on all routers in a single OSPF area.
D.When no 'router-id' is configured, a loopback interface with the highest IP address is preferred over a physical interface for the router-id.
E.The 'show ip ospf' command displays the current OSPF router-id.
AnswersD, E

When no explicit 'router-id' command is configured, OSPF determines the router ID by first preferring the IP address of any loopback interface, choosing the highest numeric loopback address, and only then falling back to the highest IP address on a physical interface. Loopback interfaces are always up and do not depend on physical link state, making them stable and thus preferred for router ID selection. This behavior is defined by the OSPF RFC and is a well-known practical rule.

Why this answer

OSPFv2 selects the router-ID based on the highest IP address of any loopback interface when no explicit 'router-id' is configured, making D correct. The 'show ip ospf' command displays the current router-ID, verifying choice E. Option A is incorrect because the router-ID is derived from IP addresses, not MAC addresses.

Option B fails because changing the router-ID requires a reload or clearing the OSPF process to take effect. Option C is wrong because each router must have a unique router-ID; they do not need to match across the area.

Exam trap

Cisco often tests the misconception that changing the router-id takes effect immediately, but in reality, you must clear the OSPF process or reload the router for the change to apply.

Why the other options are wrong

A

The router-ID is derived from the highest IP address on a loopback or physical interface, never from a MAC address.

B

A router-id change does not take effect immediately; you must clear the OSPF process or reload the router.

C

Router-IDs must be unique per router, not identical across all routers in the area.

91
Drag & Dropmedium

Drag and drop the following steps into the correct order to configure OSPFv3 for IPv6 on a Cisco router.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4

Why this order

After entering global config, create the OSPFv3 process, set a router ID, then enable OSPFv3 on the desired interfaces under interface configuration.

Exam trap

Remember that OSPFv3 requires a router ID to be explicitly configured (or derived from an IPv4 address) before enabling it on interfaces. The process must be created first, then the router ID, then interface enablement.

92
Multi-Selectmedium

A router learns a route to 172.16.0.0/16 via OSPF (administrative distance 110) and a route to 172.16.10.0/24 via EIGRP (administrative distance 90). No other overlapping routes exist. Which TWO statements about how the router handles these routes are correct?

Select 2 answers
A.The router installs only the EIGRP route because it has a lower administrative distance.
B.Both the OSPF and EIGRP routes are installed in the routing table.
C.Traffic to 172.16.10.100 is forwarded using the OSPF route.
D.The EIGRP route is used for all traffic destined to any address within 172.16.0.0/16.
E.The OSPF route is used for destinations within 172.16.0.0/16 that are not part of the 172.16.10.0/24 subnet.
AnswersB, E

Because 172.16.0.0/16 and 172.16.10.0/24 are not identical prefixes, the router treats them as separate destinations in the routing table. The RIB can hold a less-specific covering route and a more-specific route simultaneously, regardless of administrative distance. Longest-prefix matching during packet forwarding then determines which route is used for a given destination.

Why this answer

B is correct because the router installs both routes in the routing table when they have different prefix lengths. The EIGRP route to 172.16.10.0/24 (AD 90) is more specific than the OSPF route to 172.16.0.0/16 (AD 110). The router uses the longest prefix match rule for forwarding, so both routes coexist without conflict.

Exam trap

Cisco often tests the misconception that administrative distance alone determines which route is installed, ignoring the critical role of prefix length in the longest prefix match rule.

Why the other options are wrong

A

The router does not discard the OSPF route; it installs both /16 and /24 entries because they represent different network-specific entries.

C

The traffic matches the /24 route, not the /16, so it would be forwarded via the EIGRP next-hop.

D

The /24 is a subset; traffic outside 172.16.10.0/24 matches only the /16 OSPF route.

93
MCQhard

R1 and R2 are connected via Ethernet and are configured with OSPF, but they fail to form an adjacency. Upon checking the interface configurations, you see that R1’s interface is in OSPF area 0 while R2’s interface is in area 1, and both interfaces use default timers and are in the same subnet. What is the most likely reason?

A.The OSPF areas do not match on the shared segment.
B.The routers must use the same process ID.
C.The interfaces are in different IPv4 subnets.
D.OSPF cannot run on Ethernet interfaces.
AnswerA

OSPF requires both neighbors to agree on the area ID for a given link; a mismatch prevents the formation of adjacency because the Hello packets carry the area ID and a mismatch causes the receiving router to drop them. Even if subnets and timers match, an area mismatch leaves the neighbor state stuck in Down or Attempt.

Why this answer

The most likely cause is an OSPF area mismatch on the shared segment. R1 and R2 are in the same IP subnet and use default hello/dead timers, so neither subnet mismatch nor timer mismatch is the problem. OSPF can run on Ethernet interfaces.

However, OSPF requires neighbors on the same link to agree on the area ID; here they differ, preventing adjacency. Option B is incorrect because OSPF process IDs are locally significant and do not need to match.

Exam trap

A common mistake is believing that OSPF process IDs must match between routers to form an adjacency; process IDs are locally significant and do not need to match.

Why the other options are wrong

B

This option is incorrect because OSPF process IDs are locally significant and do not need to match between routers. Different process IDs do not prevent adjacency formation.

C

This option is incorrect because the interfaces are in the same IPv4 subnet (10.1.50.0/24), which is a prerequisite for OSPF adjacency on broadcast networks.

D

This option is incorrect because OSPF commonly runs on Ethernet interfaces. Ethernet is a supported media type for OSPF neighbor discovery and adjacency formation.

94
MCQhard

Exhibit: R1 has the static route 'ip route 0.0.0.0 0.0.0.0 192.0.2.2 200' and also learns a default route from OSPF. Which default route will be installed while the OSPF route is present?

A.The static default route
C.Both default routes with per-packet load balancing
D.Neither route because floating statics suppress dynamic defaults
AnswerB

The OSPF default route is the preferred route because it has a lower administrative distance (110) than the floating static default route, which is configured with a higher AD (e.g., 200) to serve as a backup. OSPF injects the default via 'default-information originate', and the routing table installs only the OSPF route for 0.0.0.0/0. The static route remains in the configuration but is not used until the OSPF route disappears.

Why this answer

The OSPF external default route wins because its administrative distance is 110, which is lower than the floating static route's distance of 200. The static route is intentionally given a higher AD so it is used only as a backup.

Exam trap

A common exam trap is assuming that static routes always override dynamic routes because static routes have a default administrative distance of 1. However, in this question, the static route is configured with an administrative distance of 200, making it less preferred than the OSPF route with an AD of 110. This floating static route is intended as a backup and will not be installed while the OSPF route is present.

Misreading the static route’s AD or ignoring it leads to the incorrect conclusion that the static route will be used, which is a frequent mistake in CCNA routing questions.

Why the other options are wrong

A

The static default route has an administrative distance of 200, which is higher than OSPF’s 110. Therefore, it is less preferred and will not be installed while the OSPF route is present, making this option incorrect.

C

Cisco routers do not perform per-packet load balancing between routes with different administrative distances. The route with the lowest AD is installed exclusively, so this option is incorrect.

D

Floating static routes do not suppress dynamic routes; instead, they serve as backups when dynamic routes are unavailable. Therefore, neither route is not installed is incorrect because the OSPF route will be installed.

95
MCQhard

R1 and R2 should form an OSPF adjacency on their shared GigabitEthernet link, but they remain stuck in EXSTART. What is the most likely cause?

A.The routers are using different OSPF areas.
B.The interface MTU values do not match.
C.One side is configured as passive-interface.
D.The subnet mask on the link is incorrect.
AnswerB

A mismatch in interface MTU is the classic cause of OSPF neighbors becoming stuck in EXSTART or EXCHANGE. During the Database Description (DBD) packet exchange, each router includes its MTU in the DBD header; if the receiving router sees a larger MTU than its own, it drops the packet, so the neighbor state never progresses past EXSTART. This correctly explains why the adjacency fails to reach FULL.

Why this answer

The MTU values do not match. OSPF neighbors can discover each other and even move through earlier states, but an MTU mismatch commonly leaves them stuck in EXSTART or EXCHANGE because the routers do not agree on database description packet sizing. Area mismatch, network mismatch, and passive-interface issues usually prevent a much earlier stage of adjacency formation.

Exam trap

A common exam trap is selecting area mismatch or passive-interface as the cause of OSPF adjacency stuck in ExStart. While area mismatch prevents neighbor formation entirely, and passive-interface stops hello packets, these issues cause earlier failures, not ExStart stalls. The ExStart state specifically involves negotiating database description packets, which requires matching MTU values.

Candidates often overlook MTU mismatches because neighbors appear in the topology, misleading them to suspect other configuration errors. Understanding that MTU mismatch allows neighbor discovery but blocks database synchronization is key to avoiding this trap.

Why the other options are wrong

A

Both routers are configured in area 0, so area mismatch is not the cause. Area mismatches prevent neighbor discovery, which would stop adjacency formation before ExStart.

C

If one interface is passive, OSPF hello packets are not sent, preventing neighbor formation altogether. This would stop adjacency formation earlier than ExStart.

D

The subnet mask is consistent on both sides, allowing neighbor discovery. Incorrect subnet masks would prevent neighbors from recognizing each other, blocking adjacency before ExStart.

96
MCQhard

A network engineer notices that R1 is using the static route to 192.168.10.0/24 via next-hop 10.1.1.2 instead of the OSPF route via 10.2.2.2, even though the OSPF path has lower latency. What is the most likely cause?

A.The static route has a lower administrative distance than the OSPF route.
B.The OSPF route has a higher metric than the static route.
C.Equal-cost multi-path (ECMP) is disabled for OSPF.
D.The static route is configured with a higher next-hop IP address, so it is preferred.
AnswerA

Administrative distance is the first criterion used to select routes from different routing protocols. A static route has an AD of 1, while OSPF has an AD of 110. The lower AD wins, so the static route is installed in the routing table and used for forwarding, regardless of the OSPF path's better performance.

Why this answer

R1 prefers the static route over the OSPF route because static routes have a default administrative distance (AD) of 1, whereas OSPF has a default AD of 110. Since a lower AD indicates a more trustworthy route, the static route is installed in the routing table regardless of metric or latency. The OSPF route, despite having lower latency, is not used because AD is evaluated before metric.

Exam trap

Cisco often tests the distinction between administrative distance and metric, trapping candidates who assume lower latency or lower metric always determines the best path, when in fact AD is evaluated first for routes from different sources.

Why the other options are wrong

B

Common misconception that metrics are compared across different routing protocols. In reality, AD is evaluated first, and only routes from the same protocol with equal AD are compared by metric.

C

Mistaking ECMP as the mechanism for selecting between two routes from different sources; in this scenario, the static route's AD disqualifies the OSPF route entirely.

D

Misunderstanding that IP addresses, not administrative distance or metrics, influence path selection.

97
PBQmedium

You are connected to R1 via console. R1 connects three subnets: 10.0.1.0/24 (area 0), 10.0.2.0/24 (area 0), and 10.0.3.0/24 (area 0). The serial link to R2 uses IP subnet 10.0.0.0/30 and is in OSPF area 1. The network administrator wants to advertise a single summary route for these three subnets to R2, reducing the OSPF link-state database size in area 1. R1 is already running OSPF with network statements for its connected subnets in their respective areas. You need to configure route summarization on R1 so that only the summary route is advertised to R2 via the serial link.

Hints

  • Summarization in OSPF is configured under the router ospf process using the area range command.
  • The summary route must cover all subnets with a single prefix.
  • Verify that the summary route appears in the OSPF database as a type 3 summary LSA.
A.router ospf 1 area 0 range 10.0.0.0 255.255.252.0
B.router ospf 1 summary-address 10.0.0.0 255.255.252.0
C.interface serial 0/0/0 ip summary-address ospf 1 10.0.0.0 255.255.252.0
D.router ospf 1 area 0 range 10.0.0.0 255.255.255.0
AnswerA
solution
! R1
router ospf 1
area 0 range 10.0.0.0 255.255.252.0

Why this answer

The 'area 0 range' command creates a summary route for the specified range, which is then advertised as a type 3 summary LSA to other areas. This reduces routing table size and prevents flapping.

Exam trap

Be careful not to confuse OSPF summarization commands with those of other routing protocols. OSPF uses 'area range' for internal summarization and 'summary-address' for external routes, while EIGRP uses 'summary-address' on interfaces. Also, ensure you calculate the correct summary mask that covers all subnets without being too broad or too specific.

Why the other options are wrong

B

The 'summary-address' command is not valid for OSPF; it is used in EIGRP and BGP.

C

The 'ip summary-address ospf' command is used for external route summarization, not for summarizing internal OSPF routes within an area.

D

The mask 255.255.255.0 is too specific and only covers a single /24 network, not the required range.

98
PBQhard

You are connected to R1. Configure OSPFv2 on R1 and R2 so that they form a full adjacency and can exchange routes. The current configuration has mismatched hello/dead timers blocking the adjacency. Adjust only the necessary settings on R1 to match R2's OSPF timers.

Network Topology
G0/0192.0.2.1/30G0/0192.0.2.2/30linkR1R2

Hints

  • Check the OSPF interface timers on both routers using 'show ip ospf interface'.
  • The dead interval must be exactly four times the hello interval unless manually set.
  • Use the 'ip ospf hello-interval' and 'ip ospf dead-interval' commands under the interface configuration.
A.Configure 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' on R1's GigabitEthernet0/0 interface.
B.Configure 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40' on R2's interface to match R1's default timers.
C.Configure 'ip ospf hello-interval 5' on R1's GigabitEthernet0/0 interface only; the dead interval will adjust automatically.
D.Configure 'ip ospf dead-interval 20' on R1's GigabitEthernet0/0 interface only; the hello interval will adjust automatically.
AnswerA
solution
! R1
interface gigabitethernet0/0
ip ospf hello-interval 5
ip ospf dead-interval 20

Why this answer

The adjacency between R1 and R2 is not forming because the OSPF hello and dead timers are mismatched. R1 has default timers (Hello 10, Dead 40) while R2 has custom timers (Hello 5, Dead 20). To form an adjacency, OSPF timers must match on both ends.

On R1, you need to configure the OSPF interface timers to match R2 by issuing 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' on interface GigabitEthernet0/0. After applying these commands, the adjacency should come up.

Exam trap

A common trap is assuming that changing only the hello interval will automatically adjust the dead interval, or vice versa. In Cisco IOS, these timers are independent and must be set explicitly. Also, note that the dead interval must be at least four times the hello interval, but that relationship is not enforced automatically.

Why the other options are wrong

B

The specific factual error is that the instruction limits changes to R1 only; modifying R2 is not allowed.

C

The specific factual error is that the dead interval is not automatically derived from the hello interval; it must be set separately.

D

The specific factual error is that the hello interval is not automatically derived from the dead interval; both must be set explicitly.

99
PBQhard

You are connected to R1 via console. R1 and R2 are directly connected via GigabitEthernet0/0. Configure OSPF process 1 on both routers so that they form a full adjacency. R1's router-id must be 1.1.1.1, and R2's router-id must be 2.2.2.2. Use network statements to advertise the direct link. Ensure that R1 does not send OSPF hellos out of its GigabitEthernet0/1 interface. The current configuration on R1 has mismatched hello and dead timers, and an incorrect network type, preventing adjacency. Fix all issues.

Network Topology
G0/0192.0.2.1/30G0/0192.0.2.2/30linkR1R2

Hints

  • Check the hello and dead timer values on R1 vs R2 using show ip ospf interface.
  • The default hello timer for broadcast networks is 10 seconds, dead timer 40 seconds.
  • Use interface configuration mode to change OSPF timers.
A.On R1, configure 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40' under interface GigabitEthernet0/0, and ensure network type is broadcast. On R2, configure 'router-id 2.2.2.2' under router ospf 1. Also, on R1, add 'passive-interface GigabitEthernet0/1' under router ospf 1.
B.On R1, configure 'ip ospf hello-interval 30' and 'ip ospf dead-interval 120' under interface GigabitEthernet0/0, and set network type to point-to-point. On R2, configure 'router-id 2.2.2.2' under router ospf 1. Also, on R1, add 'passive-interface default' under router ospf 1.
C.On R1, configure 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40' under interface GigabitEthernet0/0, and set network type to point-to-point. On R2, configure 'router-id 2.2.2.2' under router ospf 1. Also, on R1, add 'passive-interface GigabitEthernet0/1' under router ospf 1.
D.On R1, configure 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40' under interface GigabitEthernet0/0, and ensure network type is broadcast. On R2, configure 'router-id 2.2.2.2' under router ospf 1. Also, on R1, add 'network 10.0.0.0 0.255.255.255 area 0' under router ospf 1.
AnswerA
solution
! R1
interface GigabitEthernet0/0
ip ospf hello-interval 10
ip ospf dead-interval 40

Why this answer

The adjacency was failing because R1 had incorrect hello (30) and dead (120) timers, while R2 used defaults (10/40). Additionally, R1’s network type was set to a non-broadcast type, causing a mismatch. To fix, on R1’s GigabitEthernet0/0, set hello-interval 10, dead-interval 40, and network type broadcast.

In OSPF process 1 on R1, configure passive-interface GigabitEthernet0/1 to suppress hellos on that interface. On R2, under router ospf 1, set router-id 2.2.2.2. Option A addresses all requirements.

Option B uses wrong timers and passive-interface default, which blocks hellos on all interfaces, breaking adjacency. Option C sets network type point-to-point, creating a type mismatch with R2’s broadcast, preventing adjacency. Option D omits the passive-interface command, failing to suppress hellos on GigabitEthernet0/1 as required.

Exam trap

The exam trap is that candidates may focus on the network type or extra network statements, but the primary issue is the timer mismatch. OSPF requires hello and dead timers to match for adjacency. Also, remember that 'passive-interface default' suppresses hellos on all interfaces, which would break the adjacency; use specific passive-interface commands.

Why the other options are wrong

B

Incorrect timers (30/120) break adjacency, and 'passive-interface default' suppresses hellos on all interfaces, including the transit link.

C

Network type point-to-point on R1 does not match R2's default broadcast, causing a type mismatch that prevents OSPF adjacency.

D

Omits the required 'passive-interface GigabitEthernet0/1' command, leaving hellos sending on that interface.

100
MCQhard

Two directly connected routers, R1 and R2, are configured with single-area OSPF in Area 0. The administrator notices that they are not forming a full OSPF neighbor adjacency. The exhibit displays relevant portions of the running configurations. What is the most likely cause of the problem?

A.The network command on R1 does not include the correct subnet mask.
B.R1's passive-interface default prevents OSPF hello packets from being sent on GigabitEthernet0/0.
C.The GigabitEthernet0/0 interface on R2 is administratively down.
D.The routers are configured with different OSPF area IDs.
AnswerB

R1's configuration includes `passive-interface default`, which makes all OSPF-enabled interfaces passive by default. A passive interface does not send or process OSPF hello packets, so R1 never establishes a neighbor relationship on GigabitEthernet0/0. Unless a `no passive-interface GigabitEthernet0/0` statement is present, hellos are suppressed even though the interface is up and the network statement matches.

Why this answer

The passive-interface default command on R1 sets all interfaces to passive by default, which prevents OSPF hello packets from being sent out GigabitEthernet0/0. Without hello packets, R1 cannot discover R2 or form a neighbor adjacency, even though the network command is correctly configured. This is the most likely cause because the exhibit shows R1's configuration includes passive-interface default without a corresponding no passive-interface GigabitEthernet0/0 statement.

Exam trap

Cisco often tests the passive-interface default command as a trap, because candidates may overlook that it applies to all interfaces unless explicitly overridden, leading them to incorrectly focus on network command mismatches or area ID issues.

Why the other options are wrong

A

The network statement is syntactically correct and covers the interface IP address, so it does enable OSPF process on that interface (subject to the passive-interface setting).

C

The configuration shows 'no shutdown', indicating the interface is enabled. Administrative down would require the 'shutdown' command or lack of 'no shutdown'.

D

The output clearly shows 'area 0' in both routers' OSPF configurations, so area mismatch is not the cause.

101
MCQhard

Refer to the exhibit. A network engineer is troubleshooting an OSPF adjacency issue between R1 and R2. The output of the show ip ospf neighbor command on R1 shows the neighbor relationship with R2 stuck in the EXSTART/DROTHER state. What is the most likely cause?

A.OSPF network type mismatch between R1 and R2, preventing full adjacency.
B.Duplicate OSPF Router IDs on R1 and R2, causing DBD packet rejection.
C.OSPF authentication mismatch on the link, causing DBD packets to be rejected.
D.MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.
AnswerD

The EXSTART/DROTHER state in the exhibit indicates that OSPF is stuck in the DBD exchange phase. This is a classic symptom of an MTU mismatch, where one side creates DBD packets larger than the other's MTU, leading to silent drops. The output directly confirms the neighbor is in EXSTART, not EXCHANGE or FULL.

Why this answer

The EXSTART/DROTHER state indicates that the routers have progressed beyond the 2-WAY state but are stuck during the Database Description (DBD) packet exchange phase. An MTU mismatch between R1 and R2 causes DBD packets to be dropped because the receiving router will reject packets larger than its configured interface MTU, preventing the routers from completing the master/slave election and database synchronization.

Exam trap

The trap here is that candidates often confuse the EXSTART/DROTHER state with authentication or network type mismatches, but Cisco specifically tests that MTU mismatches cause DBD packet drops during the database exchange phase, not earlier adjacency stages.

Why the other options are wrong

A

Candidates may confuse adjacency failures with DBD exchange problems, but network type mismatch leads to a different state (2-WAY).

B

Candidates think duplicate Router IDs might cause DBD exchange failure, but duplicate IDs prevent neighbor discovery entirely.

C

Many associate EXSTART with any adjacency issue, but authentication errors prevent the neighbor from being listed at all.

102
MCQmedium

A branch router learns a route to 10.20.30.0/24 from OSPF with metric 30 and also has a static route to the same prefix with an administrative distance of 5. Which route will appear in the routing table?

A.The OSPF route because metric 30 is lower than the static route metric
B.The static route because its administrative distance is lower
C.Both routes with equal preference because they point to the same prefix
D.Neither route until the router performs a full SPF recalculation
AnswerB

A static route is assigned an administrative distance of 1 by default in Cisco IOS, whereas an OSPF internal route has an AD of 110. Because the router always prefers the route with the lowest AD, it installs the static route in the routing table and uses it to forward traffic to 10.20.30.0/24. The OSPF route is retained in the OSPF database but not placed in the RIB.

Why this answer

The router installs the static route because administrative distance is compared before metric when two different routing sources advertise the same prefix. OSPF metric matters only against other OSPF choices, not against a lower-AD static route.

Exam trap

A common exam trap is to confuse the OSPF metric with administrative distance and assume the route with the lower metric is preferred. Since OSPF’s metric is 30 and the static route’s metric is not applicable or higher, candidates may incorrectly select the OSPF route. However, Cisco routers first compare administrative distance, which is a measure of route trustworthiness across different routing sources.

Because the static route has a lower administrative distance (5) than OSPF (110), the static route is preferred and installed in the routing table. Misunderstanding this leads to incorrect route selection and exam errors.

Why the other options are wrong

A

This option is incorrect because metric values are only compared among routes learned from the same routing protocol. The OSPF metric of 30 is irrelevant when compared to a static route, which is a different routing source with a lower administrative distance.

C

This option is incorrect because routers do not install multiple routes to the same prefix from different routing protocols unless they have equal administrative distance and are configured for load balancing. Here, the static route’s lower AD prevents the OSPF route from being installed.

D

This option is incorrect because the router does not delay route installation until a full SPF recalculation. Route selection is immediate based on administrative distance and metric rules. SPF recalculation only affects OSPF route selection, not cross-protocol comparisons.

103
MCQhard

R1 learns three OSPF routes to different destinations: O 10.10.10.0/24 O IA 10.20.20.0/24 O E2 10.30.30.0/24 Which statement is correct about these route types?

A.O IA is an external route redistributed from another routing protocol.
B.O E2 is an OSPF external type 2 route.
C.O means the route was learned through EIGRP.
D.All three routes were learned from the same OSPF area type.
AnswerB

In OSPF, the route code 'O' indicates a route learned via the OSPF protocol, and 'E2' specifically denotes an external type 2 route. This type is redistributed from another routing protocol into OSPF, and its metric is the external cost only, without adding the internal cost to the ASBR. The 'O E2' code is exactly the standard representation for such externally redistributed type 2 routes.

Why this answer

An O route is intra-area, O IA is interarea, and O E2 is an external type 2 route redistributed into OSPF. The codes describe route origin, not just preference. CCNA expects you to identify them quickly when reading the routing table.

Exam trap

A frequent exam trap is confusing the OSPF route codes, especially mistaking 'O IA' (inter-area) for an external route redistributed from another protocol. Candidates often incorrectly assume that 'O IA' means external, but it actually represents routes learned from a different OSPF area within the same autonomous system. Another common mistake is thinking the 'O' code indicates EIGRP routes, which it does not; EIGRP uses different codes such as 'D'.

This confusion can cause candidates to select incorrect answers about route origins or types. Carefully distinguishing between intra-area, inter-area, and external routes based on OSPF codes is essential to avoid this pitfall.

Why the other options are wrong

A

Option A incorrectly states that 'O IA' is an external route redistributed from another routing protocol. In reality, 'O IA' stands for inter-area routes within OSPF, not external routes. External routes use 'O E1' or 'O E2' codes.

C

Option C incorrectly claims that 'O' means the route was learned through EIGRP. The 'O' code is exclusive to OSPF intra-area routes. EIGRP routes use the code 'D' in Cisco routing tables.

D

Option D is incorrect because the three routes represent different OSPF route types: intra-area (O), inter-area (O IA), and external (O E2). They do not all come from the same OSPF area type.

104
PBQhard

You are connected to R1 via the console. Configure single-area OSPFv2 on R1 and R2 so that they form a full adjacency. The link between R1 and R2 uses 203.0.113.0/30. R1 has G0/0 203.0.113.1/30 and R2 has G0/0 203.0.113.2/30. R1's router-id must be 1.1.1.1, and R2's router-id must be 2.2.2.2. R1's GigabitEthernet0/0 interface is configured as a passive interface under OSPF, preventing OSPF hello messages from being sent out of that interface. Ensure that R1 does not send OSPF hellos out of its loopback0 interface (203.0.113.129/32). After configuration, verify the adjacency is established and OSPF routes are exchanged.

Hints

  • Check if G0/0 is passive on R1 using 'show ip ospf interface'
  • The passive-interface default command makes all interfaces passive unless explicitly excluded
  • Use 'no passive-interface <interface>' under router ospf to allow hellos on the link
A.The adjacency fails because R1's GigabitEthernet0/0 interface is configured as passive-interface. Remove the passive-interface command for G0/0.
B.The adjacency fails because the router-id 1.1.1.1 is not reachable from R2. Configure a static route for 1.1.1.1/32 on R2.
C.The adjacency fails because the subnet mask on the link is /30 but OSPF expects a /24. Change the mask to /24 on both interfaces.
D.The adjacency fails because OSPF is not enabled on R2's GigabitEthernet0/0 interface. Configure 'ip ospf 1 area 0' on R2's G0/0.
AnswerA
solution
! R1
router ospf 1
no passive-interface GigabitEthernet0/0

Why this answer

The adjacency fails because R1's GigabitEthernet0/0 interface is configured as passive-interface (the 'No Hellos' line in show ip ospf interface). This prevents R1 from sending OSPF hellos to R2. To fix, remove the passive-interface command for G0/0.

The loopback0 interface should remain passive. After removal, verify with 'show ip ospf neighbor' to see the neighbor state change to FULL and 'show ip route ospf' to see routes.

Exam trap

The trap is that candidates may overlook the 'passive-interface' command's effect on hello suppression. They might focus on router-id or subnet issues instead. Always check 'show ip ospf interface' for passive status when adjacency fails.

Why the other options are wrong

B

The router-id is used only for OSPF router identification and does not need to be reachable; adjacency uses interface IP addresses.

C

OSPF does not require a specific subnet mask; it uses the configured mask on the interface as the network type.

D

OSPF is enabled on R2's G0/0; the issue is on R1's side where the passive-interface prevents hellos.

105
MCQhard

Refer to the exhibit. A network engineer notices that traffic from R1 to the 10.1.0.0/16 network is taking a longer path than expected despite OSPF being the only routing protocol. The engineer examines the OSPF LSDB on R1 to investigate. Based on the output, what is the most likely cause of the suboptimal routing?

A.The reference bandwidth has been misconfigured on R1, causing the OSPF cost calculation to be inflated for some links.
B.The ABR is filtering the 10.1.0.0/16 route from Area 1 into Area 0, causing the router to recalculate the metric higher.
C.The ABR is configured with the 'area 1 range 10.1.0.0 255.255.0.0 cost 1000' command.
D.The metric-type for OSPF external routes has been set to type 2, causing the metric to be inflated to 1000 for the 10.1.0.0/16 prefix.
AnswerC

The ABR (10.1.1.1) is advertising a Type 3 summary LSA for 10.1.0.0/16 with a metric of 1000. This matches the behavior of the 'area range' command with the 'cost' keyword, which overrides the default metric calculation for inter-area summaries and injects the specified cost. The other LSAs from the same ABR use normal metrics, confirming the summary-specific configuration.

Why this answer

The OSPF LSDB output shows that the route to 10.1.0.0/16 has a metric of 1000, which is unusually high for an intra-area or inter-area route. The 'area 1 range 10.1.0.0 255.255.0.0 cost 1000' command on the ABR sets a fixed cost for the summarized route, overriding the default OSPF cost calculation. This causes R1 to see a higher metric for the summarized route, leading to suboptimal routing if a lower-cost path exists via another area or router.

Exam trap

Cisco often tests the distinction between route summarization with a fixed cost versus default OSPF cost calculation, and the trap here is that candidates may confuse the 'area range cost' command with external route metric manipulation or filtering, rather than recognizing it as a summary route cost override.

Why the other options are wrong

A

Candidates may attribute arbitrary high metrics to a bandwidth calculation error, without noticing that only one LSA is affected.

B

A misunderstanding that filtering can somehow alter the metric rather than block the advertisement completely.

D

Confusion between external and inter-area route types leads candidates to think that metric-type manipulation could affect an internal summary LSA.

106
PBQhard

You are connected to R1, a router that must establish OSPFv3 adjacency with R2 over the directly connected link G0/0. The current configuration is incomplete: OSPFv3 process is configured but not enabled on the interface, and global IPv6 unicast routing is missing. Configure R1 so that it becomes an OSPFv3 neighbor with R2 and learns the loopback route 2001:db8:1:2::/64 via OSPFv3. Then verify neighbor state and routing table.

Network Topology
G0/02001:db8:0:1::1/64G0/02001:db8:0:1::2/64linkR1R2

Hints

  • OSPFv3 requires IPv6 unicast routing to be enabled globally before it can operate.
  • OSPFv3 is enabled on an interface using the 'ipv6 ospf <process-id> area <area-id>' command.
  • Without the interface-level command, the router will not send hellos and will not form an adjacency.
A.Enable IPv6 unicast routing globally and activate OSPFv3 on GigabitEthernet0/0 with the command 'ipv6 ospf 1 area 0'.
B.Enable IPv6 unicast routing globally and configure OSPFv3 process 1 with the 'network' command under the OSPFv3 router configuration mode.
C.Enable IPv6 unicast routing globally and configure OSPFv3 process 1 with the 'router-id' command to ensure adjacency.
D.Enable IPv6 unicast routing globally and configure OSPFv3 process 1 with the 'passive-interface default' command to allow adjacency.
AnswerA
solution
! R1
ipv6 unicast-routing
interface GigabitEthernet0/0
ipv6 ospf 1 area 0

Why this answer

R1 is missing two critical configurations: global IPv6 unicast routing must be enabled with 'ipv6 unicast-routing', and OSPFv3 must be activated on GigabitEthernet0/0 using 'ipv6 ospf 1 area 0' under the interface. Without these, R1 cannot send or receive OSPFv3 hellos, so no adjacency forms and routes are not exchanged. After applying both commands, the neighbor state becomes FULL and the remote loopback appears in the IPv6 routing table.

Exam trap

The exam trap is that OSPFv3 configuration differs from OSPFv2: OSPFv3 does not use network statements under the router process; instead, it is enabled directly on the interface. Additionally, IPv6 unicast routing must be globally enabled before OSPFv3 can function. Candidates often forget one of these two steps.

Why the other options are wrong

B

The specific factual error is that OSPFv3 uses interface-level configuration, not network statements under the OSPF process.

C

The specific factual error is that setting a router ID alone does not activate OSPFv3 on an interface; OSPFv3 must be explicitly enabled on the interface.

D

The specific factual error is that 'passive-interface default' would actually prevent adjacency, not help form it.

107
Multi-Selecthard

Exhibit: An OSPFv2 adjacency between two routers on Ethernet is not forming. Which two mismatches would directly prevent the routers from becoming neighbors?

Select 2 answers
A.Different OSPF area assignments on the interfaces
B.Authentication mismatch between the interfaces
C.Different hostnames on the routers
D.Different loopback addresses used for management
E.One router using SSH version 2
AnswersA, B

When two routers on the same Ethernet segment have different OSPF area assignments, the area ID embedded in each Hello packet differs. A router receiving a Hello with a mismatched area ID discards the packet, preventing the neighbor relationship from even reaching the 2-Way state. Thus, area numbers must match exactly on the shared link for OSPFv2 adjacency to form.

Why this answer

On an OSPFv2 network, area mismatch and authentication mismatch both directly prevent adjacency formation. Mismatched timers (such as hello or dead intervals) also block adjacency on all network types, including Ethernet. In this scenario, the correct choices are area mismatch (A) and authentication mismatch (B).

Exam trap

A common exam trap is selecting options like different hostnames or loopback addresses as causes for OSPF adjacency failure. Candidates may mistakenly believe that router identification parameters affect neighbor formation. However, OSPF adjacency strictly depends on protocol parameters such as area ID and authentication.

Hostnames and loopback interfaces are used for management or router ID purposes but do not block adjacency. Misunderstanding this can lead to incorrect troubleshooting steps or exam answers, wasting valuable time and causing confusion.

Why the other options are wrong

C

Different hostnames do not affect OSPF adjacency since hostnames are used only for identification and management, not neighbor formation.

D

Different loopback addresses used for management do not influence OSPF adjacency on Ethernet interfaces, so they do not block neighbor relationships.

E

One router using SSH version 2 is unrelated to OSPF adjacency because SSH is a management protocol and does not impact routing protocol neighbor formation.

108
MCQhard

Refer to the exhibit. A network administrator is troubleshooting an OSPF adjacency on R1's GigabitEthernet0/0 interface connected directly to R2. R2 is powered on and shows correct OSPF configuration, but the adjacency is stuck in the INIT or DOWN state. Based on the output, what is the most likely cause of the failure?

A.The OSPF network type on R1's GigabitEthernet0/0 does not match the network type on R2.
B.The OSPF hello and dead timers on R1 do not match those configured on R2.
C.The GigabitEthernet0/0 interface on R1 has been assigned to a different OSPF area than R2's connected interface.
D.The OSPF passive-interface command has been applied to GigabitEthernet0/0, preventing hello packets from being sent.
AnswerD

The line 'No Hellos (Passive interface)' in the output is the direct and definitive indication that the GigabitEthernet0/0 interface has been configured with the OSPF passive-interface command. This command prevents OSPF from sending Hello packets out of this interface, and without Hellos, R1 cannot discover R2 or form an adjacency with it. Passive-interface is a common administrative choice to stop OSPF on a LAN segment while still advertising the connected subnet, but it breaks neighbor formation entirely, which is exactly the problem described in the scenario.

Why this answer

The adjacency is stuck in INIT or DOWN state, which indicates that R1 is not receiving OSPF Hello packets from R2. The passive-interface command on GigabitEthernet0/0 prevents R1 from sending Hellos, so R2 never sees R1, and R1 may see R2's Hellos (stuck in INIT) or not (stuck in DOWN). This matches the symptom exactly, as passive-interface suppresses outgoing Hellos while still allowing the interface to be advertised.

Exam trap

Cisco often tests the passive-interface command as a subtle cause of OSPF adjacency failure, where candidates mistakenly think it only affects routing updates (like in EIGRP) rather than Hello suppression in OSPF.

Why the other options are wrong

A

Candidates may think network type mismatch when they see no adjacency, but the exhibit does not indicate a mismatch and explicitly shows the passive state.

B

Timer mismatch is a classic troubleshooting trap, but the 'No Hellos (Passive interface)' message overrides any timer considerations.

C

Candidates may guess area mismatch as a cause, but the exhibit provides no evidence of it, while the passive-interface message is a direct cause.

109
PBQmedium

You are connected to R1 via console. R1 is connected to three routers (R2, R3, R4) over Ethernet links, all in OSPF area 0. Due to network topology, R1 should not become the Designated Router (DR) or Backup Designated Router (BDR) on any of its interfaces. You need to configure R1's OSPF priority appropriately to ensure it never participates in DR/BDR elections.

Network Topology
G0/010.0.0.1/24G0/110.0.1.1/24G0/210.0.2.1/24R1R2R3R4

Hints

  • The DR/BDR election is based on the highest OSPF priority, with a tiebreaker on router ID.
  • Setting the priority to 0 on an interface means the router cannot become DR or BDR.
  • The command is configured under the interface.
A.Set the OSPF priority to 0 on all interfaces of R1.
B.Set the OSPF priority to 255 on all interfaces of R1.
C.Set the OSPF priority to 1 on all interfaces of R1.
D.Set the OSPF priority to 0 on the loopback interface of R1.
AnswerA
solution
! R1
interface GigabitEthernet0/0
ip ospf priority 0
interface GigabitEthernet0/1
ip ospf priority 0
interface GigabitEthernet0/2
ip ospf priority 0

Why this answer

Setting the OSPF priority to 0 on each Ethernet interface excludes R1 from DR/BDR elections entirely, as a priority of 0 signals that the router is ineligible. In contrast, option B (priority 255) is incorrect because the highest priority makes a router the most likely to become DR/BDR, not prevent it. Option C (priority 1) allows the router to participate in elections and could still become DR/BDR if no higher-priority router exists.

Option D is wrong because setting priority to 0 on the loopback interface does not affect the physical Ethernet interfaces where elections occur.

Exam trap

Remember that only a priority of 0 prevents a router from being elected as DR or BDR. Do not confuse priority with other OSPF parameters like cost or hello interval. Also, priority must be set on the actual interfaces facing the multiaccess network, not on loopback or other logical interfaces.

Why the other options are wrong

B

A priority of 255 is the highest possible value, ensuring the router becomes the DR or BDR, not excluding it.

C

Priority 1 is the default and allows the router to be elected if it has the highest priority or Router ID.

D

Loopback interfaces are not used for DR/BDR elections; they are logical interfaces. The priority must be set on the physical interfaces connecting to other routers.

110
MCQhard

A network engineer notices that traffic from the router to server 192.168.10.5 is being sent over a slow backup link, even though the primary high-speed link is up. The routing table has an OSPF route for 192.168.10.0/24 via the primary link and a static host route to 192.168.10.5/32 via the backup link. Why is the backup link used for traffic to the server?

A.The static route has an administrative distance of 1, which is lower than OSPF's 110, so it is always preferred.
B.The router is load-balancing between the two routes, and traffic is being hashed to the backup link for this particular flow.
C.The OSPF route is not installed in the routing table because the static route has a better AD.
D.The static /32 route is a more specific match than the OSPF /24 route, so it is selected regardless of administrative distance.
AnswerD

The router selects the static /32 route because it offers the longest prefix match for destination 192.168.10.5. Even though OSPF has a higher administrative distance, prefix length is evaluated first in the forwarding decision. The /32 route exactly matches the destination, whereas the /24 route only matches the general subnet, so the more specific route is always chosen.

Why this answer

The router uses the most specific matching route in the routing table to forward traffic. The static host route to 192.168.10.5/32 has a longer prefix length (32 bits) than the OSPF route for 192.168.10.0/24 (24 bits), making it a more specific match. Even though OSPF has a higher administrative distance, the longest prefix match rule takes precedence over administrative distance when both routes are present in the routing table.

Exam trap

Cisco often tests the misconception that administrative distance is the sole factor in route selection, when in fact the longest prefix match rule is evaluated first and takes priority over AD for any routes that are already in the routing table.

Why the other options are wrong

A

Administrative distance is only compared when two routes have the same prefix length. Here, the /32 route has a longer prefix, so it is chosen first.

B

Load balancing requires routes with identical prefix lengths and metrics. The /32 and /24 routes are treated as different destinations.

C

AD is only compared when routes have the exact same prefix length. The routing table can hold multiple overlapping routes as long as they differ in prefix length.

111
MCQhard

An OSPF-enabled router has two paths to the same destination network, and both paths have the same OSPF cost. What is the most likely default behavior?

A.Install both routes and use equal-cost multipath forwarding
B.Discard both routes because OSPF cannot handle duplicates
C.Always keep only the route learned first
D.Replace both routes with a default route
AnswerA

OSPF is a link-state protocol that computes the shortest path tree using Dijkstra's algorithm, and when two or more paths to a destination have identical metrics, it intentionally installs them all as equal-cost multipath routes. These parallel next hops are placed in the routing table, and Cisco IOS uses CEF to load-balance traffic across them, maximizing bandwidth utilization and providing fast failover if one link goes down.

Why this answer

When OSPF learns two equally good paths to the same destination, the router can install both and perform equal-cost multipath forwarding. In plain language, the router does not have to throw one away simply because there are two valid answers. If the routes are truly equal from OSPF’s perspective, it can use both paths to improve resilience and share traffic.

This is a classic routing behavior question because many candidates assume the router must always choose only one best path. In reality, equal-cost multipath is a normal feature in many routing environments. The key is that the paths must be equally good according to the protocol’s metric logic.

Exam trap

A frequent exam trap is believing that OSPF must select only one best route when multiple paths have the same cost. Many candidates mistakenly think OSPF discards duplicates or keeps only the first learned route. This misunderstanding leads to incorrect answers suggesting route discarding or default route replacement.

The trap arises because some routing protocols or older implementations do not support equal-cost multipath. However, OSPF explicitly supports installing multiple equal-cost routes to improve load balancing and fault tolerance, so assuming otherwise causes errors in exam scenarios.

Why the other options are wrong

B

Incorrect because OSPF can handle multiple equal-cost routes and does not discard them; it uses all equal-cost paths to improve traffic distribution.

C

Incorrect since OSPF does not keep only the first learned route when multiple equal-cost paths exist; it installs all such routes for load balancing.

D

Incorrect because OSPF does not replace multiple valid equal-cost routes with a default route; default routes are used only when no specific routes exist.

112
MCQhard

R1 loses its route to 192.168.20.0/24 whenever R2's GigabitEthernet0/0 interface flaps. The network engineer has configured a floating static route with an administrative distance of 200. The OSPF route has an AD of 110. After R2's G0/0 interface recovers, the floating static route appears in the routing table instead of the OSPF route. What should the technician do next?

B.Check the carrier delay timers on R2's GigabitEthernet0/0 interface.
C.Clear the IP routing table and reset the OSPF process on R1.
D.Verify that the MTU on R1 and R2's GigabitEthernet0/0 interfaces match.
AnswerB

A high carrier-delay (interface debounce) timer can keep the link down for too long after a flap, delaying OSPF neighbor formation. While the interface remains down, the floating static route stays in the table. Checking this timer is a logical, non‑destructive next step.

Why this answer

When R2's GigabitEthernet0/0 interface flaps, the OSPF neighbor relationship goes down, causing R1 to lose the OSPF route. The floating static route (AD 200) then takes over. After the interface recovers, OSPF should re-establish and install its route (AD 110) over the static route.

However, if the carrier delay timer on R2's interface is set too high, the interface may not come up quickly enough for OSPF to re-converge before the floating static route is already installed and preferred. Checking and adjusting the carrier delay timer ensures that the interface state change is propagated promptly, allowing OSPF to re-establish and replace the static route.

Exam trap

Cisco often tests the concept that a floating static route can persist after a link recovers due to interface timers (carrier delay) delaying OSPF convergence, leading candidates to mistakenly focus on administrative distance adjustments or clearing the routing table.

Why the other options are wrong

A

Misunderstanding of route preference: a higher AD value does not keep a floating static installed when a better OSPF route becomes available.

C

Troubleshooting should follow the OSI model bottom‑up; immediately resetting processes skips basic interface‑level verification.

D

It targets a different root cause (OSPF adjacency failure due to MTU) that would manifest constantly, not only after interface recovery.

113
MCQhard

An engineer wants a static route to be used only if the OSPF route to the same network disappears. What should be configured?

A.A static route with lower administrative distance than OSPF
B.A static route with higher administrative distance than OSPF
C.A second OSPF route with a lower metric
D.A default route with no next hop
AnswerB

Configuring a static route with an administrative distance (AD) higher than OSPF's default AD (110) ensures it is less preferred by the router. Routers always prioritise routes with lower ADs when multiple paths to the same destination exist. This setup precisely satisfies the requirement for the static route to be used *only if* the OSPF route, which normally has a lower AD and is thus preferred, becomes unavailable in the routing table.

Why this answer

A floating static route is given a higher administrative distance than the preferred dynamic route so it stays out of the routing table unless the dynamic route is lost.

Exam trap

Remember, administrative distance determines route preference across different protocols, not metrics.

Why the other options are wrong

A

This option is wrong because a static route with a lower administrative distance than OSPF would take precedence over OSPF routes, making it active even when OSPF is available, contrary to the requirement of using the static route only when the OSPF route is unavailable.

C

This option is wrong because a second OSPF route with a lower metric would not serve as a backup to the existing OSPF route; instead, it would be preferred over the static route, which contradicts the requirement for the static route to be used only if the OSPF route disappears.

D

A default route with no next hop would not serve as a backup for an OSPF route because it lacks specificity and cannot direct traffic to a specific network. It would be ineffective in scenarios where a specific static route is needed when OSPF fails.

114
MCQhard

Refer to the exhibit. A network engineer is troubleshooting a missing route on R1. The router R3 is configured to advertise network 192.168.30.0/24 via OSPF, but the route is not present in the OSPF routing table of R1 when issuing the show ip route ospf command. What is the most likely cause?

A.R3 has not enabled OSPF on the network 192.168.30.0/24 using the network command.
B.R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor.
C.R1 has an inbound ACL applied to GigabitEthernet0/1 that blocks OSPF multicast traffic to 224.0.0.5 and 224.0.0.6.
D.R3's OSPF router ID is duplicated with another OSPF router, causing the SPF algorithm to drop routes.
AnswerB

Passive-interface default suppresses OSPF hello packets on all interfaces unless explicitly enabled. Without hellos, an adjacency cannot form, so R1 never learns the 192.168.30.0/24 route. The exhibit shows OSPF working with another neighbor, confirming the problem is isolated to R3’s interface configuration.

Why this answer

R3 has passive-interface default configured, which makes all interfaces passive by default, preventing OSPF hello packets from being sent. Without the no passive-interface command on the interface facing R1, R3 cannot form an OSPF adjacency, so R1 never learns the 192.168.30.0/24 route. This is a common misconfiguration when using passive-interface default to suppress OSPF on non-neighbor interfaces.

Exam trap

Cisco often tests the passive-interface default behavior where candidates assume that configuring OSPF with a network statement is sufficient to form adjacencies, overlooking that passive-interface default silently blocks hello packets on all interfaces unless explicitly overridden.

Why the other options are wrong

A

The question says R3 advertises the network via OSPF; a missing network command would contradict that, so the failure must be in adjacency formation.

C

The presence of other OSPF routes on the same interface disproves a blanket multicast ACL; the issue is specific to R3’s adjacency.

D

A duplicate router ID could prevent adjacency, but the scenario’s focus on passive-interface default provides a more common and direct cause for a total lack of adjacency when OSPF is correctly configured on the network.

115
MCQhard

Two directly connected routers running OSPFv3 do not form an adjacency. Both interfaces have valid IPv6 addresses and can ping each other using link-local addresses. What is the most likely cause?

A.The interfaces are assigned to different OSPFv3 areas.
B.The routers need global unicast addresses before OSPFv3 can run.
C.The router IDs must be identical before adjacency can form.
D.The link-local addresses must be learned from DHCPv6.
AnswerA

In OSPFv3 (and OSPFv2), each interface belongs to exactly one area. Two routers directly connected on the same link must have that link in the same area to become neighbors; if they differ, the Hello packets are rejected because the Area ID is carried in the OSPF header, and the neighbor relationship stays down.

Why this answer

The most likely cause is an OSPFv3 area mismatch on the interface. In practical terms, OSPFv3 still requires neighbors on the same link to agree on the area assignment, just as OSPF for IPv4 does. Link-local reachability alone is not enough to form an adjacency. The protocol parameters still have to match.

This is an important IPv6 routing point because people sometimes assume that successful IPv6 ping means the routing protocol should automatically work. It does not. Adjacency depends on protocol alignment, not just basic connectivity.

Exam trap

A frequent exam trap is to assume that because two routers can ping each other using IPv6 link-local addresses, their OSPFv3 adjacency should automatically form. This mistake overlooks the critical requirement that both routers must be configured in the same OSPFv3 area. Candidates might also incorrectly believe that global unicast addresses are necessary for OSPFv3 adjacency or that router IDs must be identical.

These misconceptions lead to selecting incorrect answers, as adjacency depends on matching area IDs and unique router IDs, not on global addressing or identical IDs.

Why the other options are wrong

B

Incorrect. OSPFv3 forms adjacencies using IPv6 link-local addresses, so global unicast addresses are not mandatory for adjacency formation or neighbor discovery.

C

Incorrect. Router IDs must be unique identifiers for OSPF routers. Identical router IDs cause adjacency failure, but they do not need to be identical to form adjacency.

D

Incorrect. IPv6 link-local addresses are automatically configured on interfaces and do not require DHCPv6. OSPFv3 uses these link-local addresses for neighbor communication.

116
PBQhard

You are connected to R1 via console. Configure OSPFv3 for IPv6 on R1 and R2 so that IPv6 loopback interfaces on both routers can communicate. R1's GigabitEthernet0/0 and R2's GigabitEthernet0/1 are directly connected. Ensure OSPFv3 is enabled on the correct interfaces and verify neighbors and routes.

Network Topology
G0/02001:db8:12::1/64G0/12001:db8:12::2/64linkR1R2

Hints

  • OSPFv3 must be enabled per-interface, not globally.
  • Use the same OSPFv3 process ID and area on both routers.
  • Check which interfaces are physically connected between the routers.
A.Enable OSPFv3 on R1's GigabitEthernet0/0 and Loopback0, and on R2's GigabitEthernet0/1 and Loopback0.
B.Enable OSPFv3 only on the loopback interfaces of both routers.
C.Enable OSPFv3 on R1's GigabitEthernet0/0 and R2's GigabitEthernet0/1 only, without loopbacks.
D.Enable OSPFv3 on R1's Loopback0 and R2's GigabitEthernet0/1 only.
AnswerA
solution
! R1
interface GigabitEthernet0/0
ipv6 ospf 1 area 0
interface Loopback0
ipv6 ospf 1 area 0

! R2
interface GigabitEthernet0/1
ipv6 ospf 1 area 0
interface Loopback0
ipv6 ospf 1 area 0

Why this answer

The issue is that OSPFv3 is not enabled on the interfaces. On R1, OSPFv3 must be enabled on GigabitEthernet0/0 (the link to R2) and Loopback0 (to advertise the loopback). On R2, OSPFv3 must be enabled on GigabitEthernet0/1 (the link to R1) and Loopback0.

After enabling OSPFv3 on the correct interfaces, the neighbor adjacency forms and routes are exchanged.

Exam trap

A common trap is to enable OSPFv3 only on the link interfaces or only on the loopbacks. Remember that OSPFv3 must be enabled on every interface that needs to be advertised or that participates in neighbor discovery. Also, note that OSPFv3 uses 'ipv6 ospf <process-id> area <area-id>' under the interface, not the network statement used in OSPFv2.

Why the other options are wrong

B

OSPFv3 requires the link interface to be enabled to form neighbors; loopback-only configuration results in no neighbor relationship.

C

OSPFv3 must be enabled on the loopback interfaces to advertise their prefixes; otherwise, they remain unknown to the neighbor.

D

Both routers must have OSPFv3 enabled on the link interface to form an adjacency, and both loopbacks must be enabled to advertise their prefixes.

117
MCQhard

A router has the following routes in its routing table: a static route to 10.60.4.16/28, an OSPF route to 10.60.4.0/24, and an EIGRP route to 10.60.0.0/16. Which route will be used for a packet destined to 10.60.4.17?

A.The static route to 10.60.4.16/28
B.The OSPF route to 10.60.4.0/24
C.The EIGRP route to 10.60.0.0/16
D.No route at all
AnswerA

The destination address 10.60.4.17 falls within all three advertised prefixes, but the router selects the route with the longest prefix match. The static route to 10.60.4.16/28 uses a 28-bit subnet mask, which is longer than the 24-bit OSPF route and the 16-bit EIGRP route. Because this /28 prefix provides the most specific match for the destination, it overrides the other less-specific entries in the routing table.

Why this answer

The route used will be the route with the longest matching prefix. In practical terms, 10.60.4.17 falls inside the /28 route shown, and that is more specific than the broader /24 and /16 alternatives. Because specificity comes first, the /28 route wins.

This is a clean route-table interpretation problem that mirrors actual exam-style thinking very closely.

Exam trap

Remember, the longest prefix match rule is crucial; don't assume broader prefixes are better.

Why the other options are wrong

B

This option is wrong because the OSPF route to 10.60.4.0/24 does not directly match the destination 10.60.4.17, which falls under the static route to 10.60.4.16/28. OSPF would not be preferred if a more specific static route exists.

C

The EIGRP route to 10.60.0.0/16 is not used for the destination 10.60.4.17 because it does not match the more specific subnet of 10.60.4.16/28, which is preferred in routing decisions.

D

This option is wrong because there is a valid static route to the destination 10.60.4.17 via the static route to 10.60.4.16/28, making it reachable. Therefore, stating 'no route at all' is incorrect in this context.

118
MCQhard

A network engineer configures OSPF between R1 and R2, but the routers never become neighbors on GigabitEthernet0/0. Based on the exhibit, what is the most likely cause?

A.The routers are using different OSPF areas.
B.The OSPF interface timers do not match.
C.The routers must use the same hostname to become neighbors.
D.The subnet mask prevents multicast OSPF traffic.
AnswerB

OSPF requires that neighbor routers on the same link agree on both the hello interval and the dead interval. If R1 uses the default 10-second hello and 40-second dead timers while R2 has modified those timers, the routers will not recognize each other as valid neighbors and will not exchange OSPF packets. This timer mismatch prevents adjacency formation, regardless of any other matching parameters.

Why this answer

The most likely cause is a hello/dead timer mismatch. In practical terms, both routers are on the same subnet and both are trying to run OSPF in area 0, so the obvious items look correct. But OSPF does not form adjacency based on IP reachability alone. It also checks whether key interface parameters agree. Hello and dead intervals are among those required parameters.

On R1, the hello interval is 10 seconds and the dead interval is 40 seconds. On R2, the hello interval is 5 seconds and the dead interval is 20 seconds. That mismatch is enough to prevent the neighbor relationship from forming. This is a common CCNA troubleshooting pattern because the interfaces can still ping each other, which makes the failure look less obvious at first glance.

Exam trap

A frequent exam trap is assuming that OSPF adjacency forms as long as routers are in the same area and can ping each other. Candidates often overlook the necessity for matching hello and dead intervals, which are critical for neighbor discovery and maintenance. Because routers can still exchange ICMP packets, it may appear that the link is fully operational, leading to the mistaken belief that OSPF should work.

This misunderstanding causes candidates to select incorrect answers related to area mismatches or subnet masks, ignoring the timer mismatch that actually prevents adjacency.

Why the other options are wrong

A

This option is incorrect because both routers are configured to use the same OSPF area 0, so area mismatch is not the cause of adjacency failure.

C

Hostnames do not influence OSPF neighbor relationships; routers form adjacencies based on protocol parameters, not device names, so this option is invalid.

D

A /24 subnet mask does not block OSPF multicast traffic; OSPF uses multicast addresses that function normally on standard subnet masks, so this option is incorrect.

119
PBQhard

You are connected to R1. R1 has OSPF configured on GigabitEthernet0/0 with network 10.0.0.0 0.0.0.3 area 1, ip ospf hello-interval 10, and ip ospf dead-interval 40. R2 has OSPF configured on its GigabitEthernet0/0 with network 10.0.0.0 0.0.0.3 area 0, ip ospf hello-interval 5, and ip ospf dead-interval 20. Correct these mismatches so that R1 and R2 become OSPF neighbors.

Network Topology
G0/010.0.0.1/30G0/010.0.0.2/30linkR1R2

Hints

  • Check the OSPF area configuration on both routers.
  • Use 'show ip ospf interface' to compare hello and dead timers.
  • The timers must match exactly on both sides of the link.
A.On R1, change the network statement to area 0 and set ip ospf hello-interval 5 and ip ospf dead-interval 20 under the interface.
B.On R2, change the network statement to area 1 and set ip ospf hello-interval 10 and ip ospf dead-interval 40 under the interface.
C.On R1, change the network statement to area 0 and set ip ospf hello-interval 10 and ip ospf dead-interval 40 under the interface.
D.On R2, change the network statement to area 0 and set ip ospf hello-interval 10 and ip ospf dead-interval 40 under the interface.
AnswerA
solution
! R1
configure terminal
router ospf 1
no network 10.0.0.0 0.0.0.3 area 1
network 10.0.0.0 0.0.0.3 area 0
interface gigabitethernet 0/0
ip ospf hello-interval 5
ip ospf dead-interval 20
end

Why this answer

The OSPF adjacency fails due to two mismatches. First, the area is mismatched: R1 uses area 1, R2 uses area 0 (both must be the same area, typically area 0). Second, the hello and dead timers are mismatched: R1 uses Hello 10/Dead 40, R2 uses Hello 5/Dead 20.

To fix, on R1 change the network statement to area 0 and adjust timers to match R2 (or vice versa). The solution below changes R1's area to 0 and sets hello to 5 and dead to 20 to match R2.

Exam trap

The exam trap is that candidates may focus on only one mismatch (area or timers) and forget that both must match. Additionally, they might assume that the router they are connected to should not be changed, or that default timers are always correct. Always verify all OSPF parameters that must match: area ID, hello/dead intervals, network type, and authentication.

Why the other options are wrong

B

The specific factual error is that the question implies you are connected to R1, so configuring R1 is the expected action. Additionally, using area 1 instead of area 0 is not recommended for a single-area design.

C

The specific factual error is that the timers are not changed to match R2; they remain at the original values.

D

The specific factual error is that R2's area is already 0, so changing it to area 0 is redundant. Changing R2's timers to match R1's original timers would still leave a mismatch because R1's timers are not changed.

120
MCQhard

A router has a static route to 10.30.5.128/25, an OSPF route to 10.30.5.0/24, and a default route 0.0.0.0/0 in its routing table. Which route will the router use for destination 10.30.5.130?

A.The static route to 10.30.5.128/25
B.The OSPF route to 10.30.5.0/24
D.No route, because the destination is outside all prefixes shown.
AnswerA

The router will utilise the static route to 10.30.5.128/25 because it adheres to the longest prefix match principle. This rule dictates that when multiple routes match a destination IP address, the router selects the route with the longest subnet mask, representing the most specific network. For 10.30.5.130, the /25 mask is more specific than the OSPF route's /24 mask or the default route's /0 mask, ensuring it is prioritised for forwarding.

Why this answer

The router selects the route with the longest prefix match between the destination IP and the prefixes in the routing table. 10.30.5.130 matches both 10.30.5.0/24 (prefix length 24) and 10.30.5.128/25 (prefix length 25); the /25 is more specific and wins. Option D is wrong because 10.30.5.130 falls within the subnet range of 10.30.5.128/25 (hosts .129–.254), so a valid route does exist.

Exam trap

Always prioritize the longest prefix match over broader subnets or default routes.

Why the other options are wrong

B

The OSPF route to 10.30.5.0/24 is less specific than the /25 static route, so the router prefers the longest mask.

C

The default route (0.0.0.0/0) has the shortest prefix length and would be used only if no other route matches.

D

10.30.5.130 is inside the 10.30.5.128/25 subnet range, meaning a route exists for this destination.

121
Multi-Selectmedium

Which TWO statements are true about OSPFv2 neighbor adjacency, network statements, and passive interfaces?

Select 2 answers
A.Configuring an interface as passive stops OSPF from sending Hello packets out of that interface, but OSPF will still advertise the connected network in its LSAs.
B.For two OSPF routers to establish full adjacency, they must agree on the Hello and Dead intervals.
C.The network command with a wildcard mask of 0.0.0.0 will enable OSPF on the exact matching interface, but it will also prevent OSPF from forming adjacencies on that interface.
D.Using the passive-interface default command makes all OSPF interfaces passive, which means OSPF will not advertise any connected networks.
E.The network command with a wildcard mask of 0.0.0.255 area 0 will match any interface whose IP address falls within that /24 subnet, but OSPF will only attempt to form adjacencies on the interface with the highest IP address in that range.
AnswersA, B

The passive-interface command disables Hello transmission on an interface, so no OSPF neighbor can ever be discovered or formed on that segment. However, OSPF's SPF calculation still requires the connected subnet to be known, so the router advertises that network as a stub (Type 1 LSA) within its area. This ensures that remote routers have a route to the subnet, even though no neighbor adjacency exists on the passive interface.

Why this answer

Configuring an interface as passive in OSPFv2 prevents the router from sending Hello packets out of that interface, which stops neighbor discovery and adjacency formation. However, OSPF still includes the connected network of that passive interface in its Type 1 Router LSAs, allowing the network to be advertised to other OSPF routers.

Exam trap

Cisco often tests the misconception that a passive interface stops network advertisement, when in fact it only stops Hello packets and adjacency formation while still advertising the connected network in LSAs.

Why the other options are wrong

C

A wildcard mask of 0.0.0.0 only matches one exact IP address. It does not impose any restriction on adjacency.

D

Passive interfaces still advertise their networks—they just do not send or receive Hello packets. Only adjacency is suppressed.

E

The network command matches multiple interfaces; OSPF then tries to form adjacencies on every enabled interface, regardless of IP address order.

122
MCQmedium

A router learns 172.16.40.0/24 from OSPF with AD 110 and metric 20. It also learns the same prefix from EIGRP with AD 90 and feasible distance 30720. Which route is installed?

A.The OSPF route because metric 20 is lower than 30720
B.The EIGRP route because its administrative distance is lower
C.Both routes because ECMP is automatic across different protocols
D.Neither route until one is redistributed into the other protocol
AnswerB

The EIGRP route is installed because Cisco routers compare administrative distance (AD) before any metric when two different routing protocols advertise the same prefix. OSPF has a default AD of 110, while EIGRP's default AD is 90, making EIGRP more trustworthy. Since the router selects the route with the lowest AD, the EIGRP route wins regardless of its metric value. Metrics are only compared among routes from the same protocol.

Why this answer

When the same prefix is learned from different routing protocols, the router compares administrative distance first. EIGRP internal routes have AD 90, which beats OSPF AD 110, so the EIGRP path goes into the table.

Exam trap

A frequent exam trap is assuming that the route with the lower metric value is always preferred, regardless of the routing protocol. In this question, the OSPF route has a metric of 20, which seems better than EIGRP's feasible distance of 30720. However, Cisco routers do not compare metrics across different protocols.

Instead, they rely on administrative distance to choose the best route. Mistaking metric comparison for cross-protocol route selection can lead to choosing the wrong answer and misunderstanding how Cisco routing decisions work.

Why the other options are wrong

A

This option incorrectly assumes that the router compares OSPF metric 20 directly against EIGRP feasible distance 30720 to select the route. Cisco routers do not compare metrics across different routing protocols; they use administrative distance first. Therefore, this option is incorrect.

C

This option incorrectly claims that both routes are installed due to automatic ECMP across different protocols. Cisco routers do not perform equal-cost load balancing between different routing protocols without explicit redistribution or configuration, so this is false.

D

This option incorrectly states that neither route is installed until redistribution occurs. Redistribution is not necessary for a router to install a route learned from a routing protocol. The router installs the route with the lowest administrative distance immediately, so this option is incorrect.

123
MCQmedium

A router interface is configured for OSPF, but neighbors do not form. The engineer checks the interface and sees Hello 10 and Dead 40. The neighbor on the same segment uses Hello 30 and Dead 120. What is the most likely cause of the OSPF adjacency failure?

A.Router IDs are identical
B.OSPF timers do not match
C.The interface cost is too low
D.The subnet mask is invalid because it is a transit link
AnswerB

Correct. This is correct. OSPF neighbors must agree on timer values such as hello and dead intervals. If they do not, the devices will not form a full adjacency even if other parts of the link appear healthy.

Why this answer

The adjacency fails because the OSPF hello and dead timers do not match. OSPF neighbors on the same segment must agree on several key parameters before they accept each other as neighbors, and the timer settings are one of those required matches. In plain terms, each router is expecting to hear OSPF hellos at one rhythm, but the other side is speaking at a different rhythm, so the relationship never becomes valid.

This is different from cost, which matters later during path selection after neighbors are already exchanging information. The timer mismatch is a classic CCNA troubleshooting point because the interfaces can be up and IP connectivity can even appear normal while the OSPF adjacency still fails.

Exam trap

Be cautious of confusing OSPF timer mismatches with other common OSPF configuration issues like network types or authentication.

Why the other options are wrong

A

This option is wrong because OSPF allows multiple routers to have the same Router ID, but they can still form adjacencies if other parameters are correctly configured. The issue in this scenario is related to mismatched OSPF timers, not Router IDs.

C

This option is incorrect because the interface cost does not directly affect the formation of OSPF adjacencies; it only influences the route selection process once adjacencies are established.

D

The subnet mask being invalid is not the cause of the OSPF adjacency failure in this scenario, as the issue lies with the mismatch of OSPF timers rather than an incorrect subnet mask. Both routers are on the same segment and should have compatible subnet configurations.

124
PBQhard

You are connected to R1 via the console. Configure single-area OSPFv2 on R1, R2, and R3 so that all three routers can exchange routes. The current configuration has mismatched hello/dead timers on the link between R1 and R2, and an area mismatch on the link between R2 and R3. Correct these issues and ensure OSPF adjacencies form.

Network Topology
G0/010.0.12.1/30G0/110.0.23.2/30G0/110.0.23.2/30G0/010.0.13.2/30G0/010.0.13.2/30G0/110.0.13.1/30R1R2 (G0/0 10.0.12.2/30)R3 (G0/1 10.0.23.2/30)

Hints

  • Check the hello/dead timers on R2's G0/0 using 'show ip ospf interface'.
  • Check the OSPF area on R2's G0/1 and R3's G0/1 using 'show ip ospf interface'.
  • On R2, use 'ip ospf hello-interval' and 'ip ospf dead-interval' under G0/0, and 'network' command under router ospf to change area.
A.On R2, configure interface G0/0 with 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40', and modify the OSPF network statement for 10.0.23.0/30 to area 0.
B.On R2, configure interface G0/0 with 'ip ospf hello-interval 30' and 'ip ospf dead-interval 120', and modify the OSPF network statement for 10.0.23.0/30 to area 1.
C.On R1, configure interface G0/0 with 'ip ospf hello-interval 30' and 'ip ospf dead-interval 120', and on R3, change the OSPF network statement for 10.0.23.0/30 to area 1.
D.On R1, configure interface G0/0 with 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40', and on R2, modify the OSPF network statement for 10.0.23.0/30 to area 1.
AnswerA
solution
! R1

Why this answer

The adjacency between R1 and R2 fails because R1 has hello/dead timers of 10/40, while R2 has 30/120 on G0/0. The adjacency between R2 and R3 fails because R2's G0/1 is in area 1, but R3's G0/1 is in area 0. On R2, change the hello/dead timers on G0/0 to match R1 (10/40) using 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40'.

Also change the network statement for 10.0.23.0/30 to area 0 using 'network 10.0.23.0 0.0.0.3 area 0'. Verify with 'show ip ospf neighbor' on all routers.

Exam trap

A common trap is to assume that timers must be changed on the router with the 'non-default' values, but OSPF requires timers to match on both ends; either router can be adjusted. Another trap is to think that the area mismatch can be resolved by changing the area on either router, but the correct area must be consistent with the network design. In this scenario, the existing explanation specifies the correction on R2.

Why the other options are wrong

B

The specific factual error is that the timers should be made consistent by adjusting R2 to match R1, not the other way around, and the area mismatch requires both routers to be in the same area, which is area 0.

C

The specific factual error is that the timers can be adjusted on either router, but the area mismatch must be resolved by making both routers use the same area. Changing R3 to area 1 would not fix the adjacency if R2 is already in area 1? Actually, R2 is in area 1 on G0/1, so changing R3 to area 1 would make them match, but the question states the area mismatch is on the link between R2 and R3, and the existing explanation says to change R2's network statement to area 0. This option changes R3 instead, which could also work, but the question expects the correction on R2 as per the existing explanation.

Additionally, changing R1's timers is not the most efficient if R1 is already configured correctly.

D

The specific factual error is that the timers on R1 are already correct, and the area mismatch requires both routers to be in the same area. Changing R2's area to area 1 would not match R3's area 0.

125
Multi-Selectmedium

Which two statements accurately describe passive interfaces in OSPF?

Select 2 answers
A.It suppresses OSPF hello packets on that interface.
B.It can still allow the connected network to be advertised into OSPF.
C.It converts OSPF on that interface into static routing.
D.It forces the interface IP address to become the router ID.
E.It disables OSPF on every interface in the router automatically.
AnswersA, B

Passive-interface in OSPF stops the router from sending or processing OSPF hello packets on that specific interface. Because hello packets are how neighbor adjacencies are discovered and maintained, the router will not form OSPF neighbors on that link, effectively preventing dynamic neighbor relationships while leaving the interface operational.

Why this answer

A passive interface stops the router from sending OSPF hello packets on that interface, which means the router will not try to form neighbors there. In practical terms, this is useful on user-facing or stub-like links where no OSPF neighbor should exist. However, passive does not necessarily mean the connected network disappears from OSPF. The network can still be advertised through the router’s other active adjacencies.

This is a common design and security practice because it reduces unnecessary routing chatter on interfaces that should not form adjacencies in the first place.

Exam trap

A frequent exam trap is assuming that configuring an interface as passive disables OSPF routing on that interface or converts OSPF routes into static routes. Candidates may also mistakenly believe that passive interfaces automatically disable OSPF on all router interfaces or force the interface IP to become the router ID. These misunderstandings arise because passive interfaces stop hello packets but do not disable OSPF or affect router ID selection.

Misreading this can lead to incorrect answers about OSPF behavior and network design.

Why the other options are wrong

C

Option C is incorrect because passive interfaces do not convert OSPF into static routing; OSPF remains active and continues to advertise routes.

D

Option D is incorrect because passive interface configuration does not influence router ID selection, which is determined by other OSPF rules.

E

Option E is incorrect because passive interface settings apply only to the specified interface and do not disable OSPF on all router interfaces.

126
MCQhard

Exhibit: R3 learns 10.50.0.0/16 by OSPF through two equal-cost paths. What will R3 do by default?

A.Install both routes and load balance
B.Choose the route from the neighbor with the highest router ID only
C.Install only one route because OSPF does not support ECMP
D.Prefer the path with the lower next-hop IP address
AnswerA

OSPF's cost-based shortest path algorithm can yield multiple paths with identical metric to the same destination. By default, the router installs all such equal-cost paths into its routing table, up to the maximum-paths limit, and load balances traffic across them. Since both paths are equal cost, the router does not pick one over the other; it uses them together for traffic sharing.

Why this answer

When OSPF has equal-cost routes to the same destination, the router installs multiple paths and can load balance across them, assuming the platform supports the default maximum-paths value.

Exam trap

A frequent exam trap is believing that OSPF will choose only one route when multiple equal-cost paths exist, often assuming it picks the route from the neighbor with the highest router ID or the path with the lowest next-hop IP address. This misconception leads to incorrect answers because OSPF’s path selection process installs all equal-cost routes by default. Misunderstanding this behavior can cause candidates to overlook OSPF’s load balancing capability and mistakenly select options that describe single-route preferences, which do not apply to equal-cost multipath scenarios.

Why the other options are wrong

B

Incorrect because OSPF does not select a single route based on the highest router ID when multiple equal-cost paths exist; it installs all such routes.

C

Incorrect because OSPF explicitly supports equal-cost multipath (ECMP) routing and does not limit itself to installing only one route.

D

Incorrect because the next-hop IP address is not a factor in OSPF’s path selection among equal-cost routes; cost metric is the deciding factor.

127
Multi-Selectmedium

A network engineer needs a floating static route to back up an OSPF-learned route. Which two configurations are necessary for the static route to remain unused until OSPF fails?

Select 2 answers
A.Configure the static route with an administrative distance higher than 110
B.Point the static route toward a valid next-hop or exit interface
C.Set the static route metric lower than the OSPF cost
D.Configure the route as directly connected with no next-hop information
AnswersA, B

A floating static route must be less preferred than the OSPF-learned route while OSPF is operational. Since OSPF has an administrative distance of 110, the competing static route should be configured with an AD greater than 110 (e.g., 120), making it a backup that is used only when OSPF disappears. This allows the route to remain in the configuration but not in the routing table during normal OSPF operation.

Why this answer

A floating static route requires an administrative distance higher than OSPF's default of 110 (option A) so that the static route is less preferred and remains inactive while OSPF is available. Additionally, the static route must point to a valid next-hop IP or exit interface (option B); without a valid next-hop, the route cannot be installed in the routing table and would never serve as a backup. Option C is incorrect because Cisco routers compare administrative distance before metric when routes come from different sources, so a lower metric cannot override the AD difference.

Option D is wrong because a directly connected route (with no next-hop) would be treated as a connected route, not a static route, and would be preferred over OSPF due to its AD of 0, defeating the purpose of a floating backup.

Exam trap

A common mistake is assuming that lowering the static route's metric makes it preferred, but Cisco routers compare administrative distance before metric when routes originate from different sources.

Why the other options are wrong

C

Setting the static route metric lower than the OSPF cost is ineffective because Cisco routers compare administrative distance first across different routing sources.

D

Configuring the route as directly connected with no next-hop information would give it an administrative distance of 0, making it preferred over OSPF, which is not the behavior of a floating static route.

128
MCQmedium

Exhibit: R1 has learned 10.50.0.0/24 through OSPF and also has a floating static route to the same prefix with administrative distance 130. Which route is installed while OSPF is healthy?

A.The static route because static routes always win over dynamic routes
B.The OSPF route because its administrative distance is lower than the floating static route
C.Both routes because the prefix length matches
D.Neither route until equal-cost load balancing is configured
AnswerB

The OSPF route is preferred because Cisco routers select the route with the lowest administrative distance when multiple routing protocols or static routes provide the same prefix. OSPF has an AD of 110, while the floating static route is intentionally configured with an AD of 130 (higher than the default static AD of 1) to act as a backup. Since 110 < 130, the router installs the OSPF route in the routing table and keeps the static route in standby until OSPF fails.

Why this answer

A floating static route is meant to stay out of the table until the preferred route disappears. OSPF has administrative distance 110, which is lower than the static route AD 130, so the OSPF route is installed while the OSPF path is still present.

Exam trap

Remember that administrative distance determines route preference. A lower value means higher preference.

Why the other options are wrong

A

This option is incorrect because static routes do not always win over dynamic routes; the route with the lowest administrative distance is preferred. In this case, OSPF has a lower administrative distance (110) than the floating static route (130).

C

This option is incorrect because only one route can be installed in the routing table for a specific prefix, and in this case, OSPF has a lower administrative distance than the floating static route, so only the OSPF route will be installed.

D

This option is incorrect because OSPF is healthy and has a lower administrative distance than the floating static route, meaning the OSPF route will be preferred and installed in the routing table.

129
MCQhard

Two routers are in the same OSPF area and on the same subnet, but they do not form an adjacency. One interface uses a hello interval of 10 seconds and the other uses 5 seconds. What is the most likely cause?

A.The OSPF hello timers do not match.
B.The routers need matching hostnames.
C.The subnet must be changed to /24 before OSPF can work.
D.The routers must both use static routes first.
AnswerA

OSPF requires neighboring routers to agree on hello and dead intervals to form and maintain adjacency. If hello timers do not match, the router will not accept the neighbor's Hello packets within the dead interval, causing the Dead Timer to expire and preventing adjacency. This specific mismatch is a classic cause when two routers are in the same area and subnet but cannot see each other as OSPF neighbors.

Why this answer

An OSPF timer mismatch is the most likely cause. In plain language, the routers are trying to discover each other, but they are speaking on different timing expectations. OSPF neighbors on the same segment must agree on certain parameters before they trust each other enough to form an adjacency, and the hello and dead timers are among those critical values.

This is a very common OSPF troubleshooting issue because the interfaces may still be reachable with ping and may even appear correctly addressed and placed in the same area. But OSPF is stricter than simple IP reachability. If the hello interval does not match, the adjacency usually fails before it becomes fully operational.

Exam trap

A common exam trap is assuming that because two routers can ping each other on the same subnet, their OSPF adjacency will form automatically. The trap lies in overlooking the importance of matching OSPF hello and dead timers. Many candidates focus only on IP addressing or area configuration and miss that OSPF requires exact timer agreement.

This leads to confusion when adjacency never forms despite correct IP setup. The exam tests your understanding that OSPF is a protocol with strict neighbor parameter requirements, not just IP reachability.

Why the other options are wrong

B

Incorrect because router hostnames do not affect OSPF adjacency formation. OSPF neighbors identify each other by router IDs and interface parameters, not hostnames.

C

Incorrect because OSPF supports various subnet masks and does not require a /24 subnet. The problem is related to timer mismatch, not subnet size.

D

Incorrect because static routes are not necessary for OSPF adjacency. OSPF dynamically discovers neighbors using hello packets and forms adjacencies based on protocol parameters.

130
Multi-Selectmedium

A branch router is running single-area OSPF. An engineer wants an interface to advertise its connected network into OSPF but must prevent hello packets from being sent on that LAN segment. Which two actions achieve that goal?

Select 2 answers
A.Enable OSPF on the interface or matching network statement
B.Configure the interface as passive in the OSPF process
C.Set the interface network type to point-to-point
D.Apply distribute-list out on the interface
AnswersA, B

Enabling OSPF on an interface directly using `ip ospf <process-id> area <area-id>` or via a `network` statement under the OSPF process configuration ensures the connected network is advertised into the OSPF routing domain. This action directly satisfies the requirement to advertise the interface's connected network into OSPF, forming one part of the overall solution.

Why this answer

In OSPF, a passive interface still advertises the connected network but does not send or process hello packets. So the interface must participate in OSPF, and then it must be made passive.

Exam trap

A frequent exam trap is assuming that changing the OSPF network type or applying a distribute-list on the interface will stop hello packets. Many candidates mistakenly believe that setting the interface to point-to-point suppresses hellos, but it only changes adjacency behavior. Similarly, distribute-lists filter routing updates but do not affect hello packet transmission.

The key mistake is forgetting that OSPF must be enabled on the interface to advertise the network and that only the passive-interface command prevents hello packets while still advertising the connected subnet.

Why the other options are wrong

C

Setting the interface network type to point-to-point changes adjacency behavior but does not suppress hello packets. Therefore, it does not meet the requirement to prevent hello packet transmission.

D

Applying a distribute-list out on the interface filters routing updates but does not affect the sending of OSPF hello packets. It does not prevent hello packet transmission on the LAN segment.

131
MCQhard

On a broadcast multiaccess segment, R3 has an OSPF priority of 255, but it is in the DROTHER state. Which explanation best fits OSPF behavior?

A.DR election is nonpreemptive on broadcast networks
B.A router cannot become DR if it uses a loopback router ID
C.OSPF priority works only on point-to-point links
D.R3's priority is lower than the current DR's priority
AnswerA

DR/BDR election is nonpreemptive on broadcast multiaccess networks: once the DR and BDR are elected, they remain in those roles until they fail or the OSPF process/interface is reset. Increasing a router's OSPF priority after the election (as with R3) does not force a new election, so the existing DR stays active even if R3 now has a higher priority. To replace the DR, you must clear the OSPF process or reload the router, not just adjust priority.

Why this answer

On broadcast networks, OSPF DR and BDR election is not preemptive. If a DR is already elected, giving another router a higher priority later does not force a new election unless the current DR goes down or OSPF is reset on the segment.

Exam trap

A common exam trap is assuming that increasing a router's OSPF priority after the DR election will cause it to immediately become the new DR. Many candidates mistakenly believe OSPF DR election is preemptive and that the router with the highest priority always becomes DR instantly. However, OSPF DR election on broadcast networks is nonpreemptive, meaning the current DR remains until it fails or OSPF adjacency resets.

This misunderstanding can lead to incorrect answers about why a router with a higher priority remains DROTHER, as in the case of R3 in the question.

Why the other options are wrong

D

This distractor assumes the election is based on current priority values, but OSPF DR election is nonpreemptive; even if R3's priority is higher, it will not become DR unless the current DR fails.

132
Drag & Dropmedium

Drag and drop the following steps into the correct order to configure single‑area OSPFv2 on a router, advertise the 192.168.10.0/24 and 10.0.0.0/24 networks in area 0, and set the GigabitEthernet0/0 interface as passive.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5
6Step 6

Why this order

The correct sequence is: (1) Enter global configuration mode with 'configure terminal' – this is required before any configuration commands. (2) Initiate the OSPF process with 'router ospf 1' – this places the CLI into OSPF router configuration mode where the remaining commands are issued. (3) Advertise the 192.168.10.0/24 network in area 0 – the 'network' command must be issued under OSPF router mode to inject the connected network into OSPF. (4) Advertise the 10.0.0.0/24 network – a second 'network' statement; the order of network commands is interchangeable but they must be configured before setting passive interfaces in a logical workflow. (5) Configure GigabitEthernet0/0 as a passive interface – this prevents OSPF Hello packets and neighbor adjacency on that interface while still advertising its subnet, and it is configured under OSPF router mode. (6) Return to privileged EXEC mode with 'end' – exits configuration mode and returns to the enable prompt. While setting a passive interface before the network statement does not break the configuration, Cisco documentation suggests adding networks first to clearly define which interfaces participate in OSPF before applying passive-interface restrictions.

133
PBQhard

You are connected to R1 via console. R1 and R2 are connected via two serial links: Serial0/0/0 (10.0.0.1/30) and Serial0/0/1 (10.0.0.5/30). OSPF is configured on both links. However, the OSPF neighbor adjacency is stuck in EXSTART/EXCHANGE state. You suspect a mismatch in OSPF parameters. You need to identify and fix the issue.

Network Topology
S0/0/010.0.0.1/30S0/0/010.0.0.2/30R1R2

Hints

  • EXSTART/EXCHANGE state indicates a problem with the Database Descriptor (DBD) packet exchange.
  • Check the MTU on the interfaces; a mismatch can cause this issue.
  • Use show ip interface to verify the MTU value.
A.Check and adjust the MTU on the serial interfaces to match.
B.Check and adjust the OSPF hello and dead timers to match.
C.Check and adjust the OSPF network type on the interfaces to match.
D.Check and adjust the OSPF area ID on the interfaces to match.
AnswerA
solution
! R1
interface Serial0/0/0
ip mtu 1400

! R2
interface Serial0/0/0
ip mtu 1400

Why this answer

The adjacency stuck in EXSTART/EXCHANGE is often due to an MTU mismatch. If one interface has a lower MTU, the DBD packets may be fragmented or rejected, preventing the exchange of LSAs. Setting the same MTU on both sides resolves the issue.

Exam trap

Do not confuse the symptoms of MTU mismatch with other OSPF parameter mismatches. MTU mismatch specifically causes problems in EXSTART/EXCHANGE, while timer mismatches cause issues in INIT/2-WAY, and area ID mismatches prevent adjacency entirely.

Why the other options are wrong

B

The specific factual error is that timer mismatches affect the neighbor discovery phase, not the database exchange phase.

C

The specific factual error is that network type affects the election of DR/BDR and adjacency formation, but not the DBD exchange process.

D

The specific factual error is that area ID mismatches cause OSPF to ignore hello packets, so the adjacency never progresses beyond DOWN.

134
MCQhard

A network engineer is troubleshooting an OSPFv3 adjacency issue between two directly connected routers. Both routers are configured for OSPFv3 in area 0 on their GigabitEthernet0/0 interfaces. The engineer checks the OSPFv3 neighbor status on R1 and sees that the neighbor state is stuck in EXSTART. The engineer verifies that both interfaces are up and have IPv6 link-local addresses. What is the most likely cause of this problem?

A.Configure a global unicast IPv6 address on the interface.
B.Ensure that the MTU is the same on both sides of the link.
C.Change the router ID to be the same on both routers.
D.Change the network type to point-to-point.
AnswerB

An MTU mismatch prevents OSPFv3 from completing the DBD packet exchange, leaving the neighbor state stuck in EXSTART. During this phase, routers send Database Description packets that must fit within the interface MTU; if one side's MTU is lower, the packets are dropped, and the routers keep retransmitting without progressing to EXCHANGE. Aligning the MTU on both ends allows the DBD exchange to complete, moving the adjacency to FULL.

Why this answer

The EXSTART state in OSPF indicates that the routers have formed a bidirectional communication and are negotiating the master/slave relationship and the initial Database Description (DBD) packet exchange. A common cause for getting stuck in EXSTART is an MTU mismatch between the two interfaces, which prevents the DBD packets from being accepted by the neighbor, causing the process to stall.

Exam trap

Cisco often tests the MTU mismatch as a specific cause for OSPF adjacency being stuck in EXSTART, and candidates may mistakenly focus on router IDs or addressing instead of the packet size negotiation.

Why the other options are wrong

A

The problem is not due to missing global addresses; the adjacency is stuck at EXSTART, not at DOWN or INIT.

C

The show output shows different router IDs (192.168.1.1 and 192.168.1.2), so this is not the issue.

D

The adjacency is stuck in EXSTART, which is not typically resolved by changing network type; the issue is related to packet exchange.

135
MCQhard

Two OSPF routers connected over Ethernet fail to become neighbors. Their interfaces are up/up and in the same IPv4 subnet. One router uses area 0 and the other uses area 1 on the connecting interfaces. What is the most likely cause?

A.Mismatched OSPF process IDs
B.Mismatched OSPF areas on the shared link
C.Missing default routes
D.Different router IDs
AnswerB

OSPF neighbors must agree on the area ID configured on the link. If one router places the interface in area 0 and the other in area 1, Hello packets are rejected because the area ID field doesn't match, preventing neighbor state from advancing beyond INIT. This is a fundamental adjacency requirement, separate from process ID or router ID issues.

Why this answer

OSPF neighbors on the same link must agree on key parameters, including the area assigned to that interface. A mismatch prevents the adjacency from forming.

Exam trap

A common exam trap is assuming that OSPF process IDs must match between neighbors for adjacency to form. Many candidates mistakenly focus on process ID alignment, but OSPF process IDs are locally significant and do not need to match. The real cause of adjacency failure in this scenario is the mismatch in OSPF area IDs on the shared link.

This subtle difference often leads to confusion, causing candidates to overlook the critical role of area consistency in neighbor formation and select incorrect answers related to process IDs or router IDs.

Why the other options are wrong

A

Mismatched OSPF process IDs do not prevent neighbor formation because process IDs are locally significant identifiers on each router. They do not need to match for adjacency to form, so this option is incorrect.

C

Missing default routes do not affect OSPF neighbor formation. Default routes influence routing decisions but are not required for establishing OSPF adjacencies, so this option is incorrect.

D

Different router IDs are necessary for OSPF neighbors to uniquely identify each router. Having different router IDs does not cause adjacency failure, so this option is incorrect.

136
Drag & Dropmedium

Drag and drop the following OSPFv2 neighbor state transitions into the correct order, starting from the initial state when no neighbor information has been received.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4

Why this order

OSPF neighbor states begin at Down, then Init after receiving Hello, 2-Way after seeing own router ID, ExStart for master/slave negotiation, and Exchange for exchanging database descriptors.

Exam trap

Do not confuse the order of ExStart and Exchange; ExStart always precedes Exchange. Also, remember that 2-Way comes after Init, not before.

137
MCQhard

Why is traffic to 10.10.10.200 using the EIGRP route instead of the OSPF route, given that both routes have the same prefix length?

A.Because EIGRP has a lower administrative distance than OSPF for the same prefix length.
B.Because OSPF routes are never installed when EIGRP is present.
C.Because OSPF can be used only for IPv6 routes.
D.Because EIGRP routes always have a smaller subnet mask than OSPF routes.
AnswerA

Both EIGRP and OSPF have learned routes to 10.10.10.0/24, so the prefix lengths are identical, meaning longest-match selection does not break the tie. Cisco's default administrative distance for internal EIGRP is 90, while OSPF's is 110; since lower AD is preferred, the EIGRP route is installed in the routing table. This is the correct explanation of why traffic to 10.10.10.200 uses the EIGRP path.

Why this answer

When two routes have the same prefix length, the router uses administrative distance to choose the route with the lowest value. EIGRP has a default administrative distance of 90, while OSPF uses 110. Therefore, the EIGRP route is preferred.

Exam trap

A common mistake is to assume that OSPF routes always have a lower administrative distance than EIGRP routes.

Why the other options are wrong

B

OSPF routes are installed when EIGRP is present; routing protocols coexist and the route with the lowest AD is selected.

C

OSPF supports both IPv4 and IPv6; it is not limited to IPv6.

D

EIGRP routes do not always have a smaller subnet mask; prefix length is determined by the network design, not the routing protocol.

138
MCQmedium

R1 receives an OSPF route to 10.55.0.0/16 and already has a static route to 10.55.10.0/24. Which route will be used for traffic sent to 10.55.10.25?

A.The OSPF /16 route, because dynamic routes override static routes learned later.
B.The static /24 route, because it is the longest-prefix match.
C.Both routes equally, because they point to the same major network.
D.Neither route, because overlapping routes are invalid.
AnswerB

The router performs a longest-prefix-match lookup in its forwarding table, and 10.55.0.0/24 has a longer prefix (more specific) than 10.55.0.0/16. Because the static route matches more bits of the destination address, it is the best route for any traffic destined to that subnet. This rule takes precedence over administrative distance or route source, so the /24 static route is always selected over the /16 OSPF route.

Why this answer

The static /24 route is more specific than the OSPF /16 route, so longest-prefix match wins. Administrative distance is only compared among routes to the same prefix length.

Exam trap

Remember that the longest-prefix match rule takes precedence over administrative distance when routes have different prefix lengths.

Why the other options are wrong

A

This option is incorrect because static routes are preferred over dynamic routes in OSPF when both are present, regardless of when they were learned. The static route to 10.55.10.0/24 will be used due to its longer prefix match.

C

This option is incorrect because OSPF routes do not share equal preference with static routes; the static /24 route will be preferred due to its longer prefix length, making it the best match for the specific destination IP.

D

This option is incorrect because overlapping routes are valid in routing protocols like OSPF, and both routes can coexist in the routing table. The static route to 10.55.10.0/24 is valid and will be preferred due to its longer prefix length.

139
MCQhard

Refer to the exhibit. A network engineer is verifying OSPF routing on R1. All routers in the topology are expected to reside in OSPF area 0, and the network 172.16.0.0/16 should be advertised from R2 within the same area. The engineer issues the show ip route 172.16.0.0 command on R1 and sees the output in the exhibit. The engineer expected to see an intra-area route (O) instead of an inter-area route (O IA). What is the most likely cause of this discrepancy?

A.R1 has a mismatched OSPF process ID compared to the ABR.
B.The network type on GigabitEthernet0/0 is configured as non-broadcast, preventing full adjacency.
C.The metric for the route is too high, causing OSPF to treat it as an inter-area route.
D.The destination network 172.16.0.0/16 is located in a different OSPF area.
AnswerD

The routing table entry shows 'type inter area'. This indicates the prefix is being learned via an OSPF Type-3 summary LSA, which is only generated when the network is in a different area than the local router. Therefore, the network is indeed in a different area, contradicting the assumption that R1 and the destination are both in area 0.

Why this answer

The route 172.16.0.0/16 appears as O IA (inter-area) on R1, which means R1 learned it via a Type 3 LSA from an ABR. For the route to be intra-area (O), the destination network must reside in the same OSPF area (area 0) as R1. Since R2 is advertising this network, if it were truly in area 0, R1 would see an O route.

The O IA designation indicates that the network is in a different area, likely because R2's interface connected to 172.16.0.0/16 is configured in a different OSPF area, or R2 is acting as an ABR.

Exam trap

Cisco often tests the distinction between intra-area and inter-area OSPF routes, and the trap here is that candidates mistakenly think a high metric or mismatched process ID can change the route type, when in fact only the area configuration determines whether a route is O or O IA.

Why the other options are wrong

A

Candidates mistakenly assume that OSPF process IDs must match globally for proper LSA exchange, but they are only locally significant.

B

Candidates may think that the network type influences the OSPF LSA type, but it only affects neighbor discovery and DR/BDR election, not the area origin of LSAs.

C

Candidates sometimes confuse the metric influence with the route type, thinking that OSPF converts routes when the cost exceeds a threshold.

140
MCQhard

R1 and R2 are connected via a shared Ethernet segment. Both routers are configured in OSPF area 0 and are on the same IP subnet. OSPF authentication is enabled on both interfaces, but the adjacency is not forming. What is the most likely reason?

A.The OSPF authentication key does not match on the two routers.
B.The routers must use different OSPF areas to become neighbors.
C.OSPF message-digest authentication can be used only on serial links.
D.The routers must remove IP addressing before OSPF can form.
AnswerA

OSPF interfaces configured with an authentication key must match the key and key ID on neighboring routers. When R1 sends a Hello packet, it computes an authentication hash using its configured key; R2 verifies that value with its own key. A mismatch causes the receiving router to silently drop the Hello, so no adjacency can form even though all other OSPF parameters are correct.

Why this answer

The most likely reason is an OSPF authentication mismatch. In practical terms, both routers are on the same IP subnet and in the same area, but they are not using the same authentication key on the shared link. OSPF neighbors must agree on key authentication parameters before they will trust each other enough to form an adjacency.

This is a classic CCNA troubleshooting pattern because the configuration looks almost correct until you compare the security settings carefully.

Exam trap

A frequent exam trap is selecting an answer that incorrectly states OSPF authentication is limited to serial links or that routers must be in different areas to form adjacency. Candidates may also mistakenly believe that removing IP addressing is necessary for OSPF to form. These misconceptions overlook that OSPF authentication applies to Ethernet interfaces and that neighbors must be in the same area and subnet with matching authentication keys.

Misreading these details leads to choosing incorrect options that seem plausible but contradict OSPF adjacency rules.

Why the other options are wrong

B

This option is incorrect because OSPF neighbors must be in the same area to form adjacency. Using different areas on the same segment prevents adjacency, so this option contradicts OSPF area rules.

C

This option is incorrect because OSPF authentication, including message-digest (MD5), is supported on Ethernet interfaces, not just serial links. Authentication is not limited by interface type.

D

This option is incorrect because OSPF requires valid IP addressing on interfaces to operate. Removing IP addressing disables OSPF on that link, so adjacency cannot form without IP addresses.

141
Drag & Dropmedium

Drag and drop the following OSPFv2 neighbor state transitions and DR/BDR election steps into the correct order for a multi-access network where a new router joins an existing OSPF area.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4

Why this order

The new router first discovers neighbors via Hello. Then DD, LSR, LSU exchange synchronizes databases. DR/BDR election happens before Full state is achieved.

Exam trap

Do not confuse the order of ExStart and Exchange, or Loading and Full. Remember that DR/BDR election occurs after 2-Way and before ExStart.

142
MCQmedium

A network engineer enters the following configuration on R1 and R2, but R1 cannot form an OSPF adjacency with R2 on interface GigabitEthernet0/0. R1# show running-config interface GigabitEthernet0/0 interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.0 ip ospf hello-interval 10 ip ospf dead-interval 40 ip ospf 1 area 0 ! R2# show running-config interface GigabitEthernet0/0 interface GigabitEthernet0/0 ip address 10.0.0.2 255.255.255.0 ip ospf hello-interval 5 ip ospf dead-interval 20 ip ospf 1 area 0 What is the most likely cause of the failure?

A.The routers are in different OSPF areas.
B.The OSPF timers on the interface do not match.
C.The subnet mask prevents multicast OSPF packets from being exchanged.
D.The OSPF process IDs do not match on the two routers.
AnswerB

This is correct because the interfaces are configured with different hello and dead intervals. OSPF neighbors expect those timers to align, and if they do not, the routers reject the neighbor relationship. The addressing and area assignment are fine, but the timer mismatch blocks adjacency formation.

Why this answer

The routers must agree on hello and dead timers to establish an OSPF adjacency. R1 uses hello=10 and dead=40, while R2 uses hello=5 and dead=20; this mismatch prevents the neighbor relationship. Although both routers share the same area (0) and subnet mask, and OSPF process IDs are locally significant (no match required), the timer difference is the root cause.

Exam trap

Remember that OSPF process IDs are locally significant and do not need to match between routers.

Why the other options are wrong

A

This option is incorrect because OSPF can still form adjacencies between routers in different areas, provided they are correctly configured to do so. The issue in this scenario is related to OSPF timers, not area mismatches.

C

This option is incorrect because OSPF can still exchange multicast packets even if the subnet mask is not configured correctly, as long as the interfaces are up and configured for OSPF. The failure to form an adjacency is more likely due to mismatched OSPF timers.

D

This option is wrong because OSPF process IDs do not need to match for routers to form an adjacency; they only need to be in the same area and have matching network statements.

143
MCQhard

An administrator has just configured OSPF in a single area between router R1 and router R2, which are directly connected via their Gi0/0 interfaces with IP addresses 10.0.0.1/30 and 10.0.0.2/30. On R1, the command show ip ospf neighbor shows no entries, and a further check on R2 with show ip ospf interface gi0/0 indicates that the interface is passive. Which configuration error is most likely causing the adjacency failure?

A.The network command on R1 does not cover the 10.0.0.0/30 subnet.
B.R2 has the passive-interface default command but no no passive-interface command for Gi0/0.
C.The OSPF process IDs on R1 and R2 are mismatched.
D.The hello and dead intervals on R1 and R2 are not the same.
AnswerB

The passive-interface default command sets all OSPF interfaces to passive mode. To allow neighbor adjacency on a specific interface, a no passive-interface <interface> command is required. Without it, Gi0/0 remains passive, preventing OSPF hellos and adjacency formation.

Why this answer

The output on R2 shows the Gi0/0 interface is passive, meaning OSPF will not send or receive hello packets on that interface, preventing neighbor discovery. The passive-interface default command makes all interfaces passive by default, and without a no passive-interface Gi0/0 command, the interface remains passive, blocking adjacency formation. This directly explains why R1's show ip ospf neighbor shows no entries.

Exam trap

Cisco often tests the distinction between passive-interface default and the need for explicit no passive-interface commands, as candidates may assume that OSPF will automatically form adjacencies on directly connected interfaces without considering passive configuration.

Why the other options are wrong

A

This option fails to account for the explicit passive interface state shown on R2. A missing network statement would not cause the interface to be displayed as passive; it would simply not be enrolled in the OSPF process.

C

This is a common misconception, but process ID mismatch does not affect OSPF neighbor formation. The passive interface status on R2 directly contradicts this as the cause.

D

The passive interface status would not appear if the only issue were interval mismatches; the interface would still be active and sending hellos. This directly conflicts with the given show output.

144
MCQhard

After configuring the area 0 range 10.0.0.0 255.255.0.0 command on an OSPF ABR, a technician finds that a host at 10.0.5.100 in Area 1 cannot reach hosts in Area 0. The ABR’s OSPF database shows only the summary 10.0.0.0/16 in Area 0, and no individual /24 routes. What is the most likely cause?

A.The ABR is filtering the specific /24 routes using a distribute-list under the OSPF process.
B.The routers in Area 1 are no longer advertising their /24 routes to the ABR because the ABR is in a different area.
C.The area range command causes the ABR to advertise only the summary LSA and suppress the more-specific Type-3 LSAs for the range.
D.The ABR has automatically created a discard route to null0 for the summary, which is dropping all traffic destined to the summarized networks.
AnswerC

By default, the area range command summarizes the specified prefix range and suppresses the individual component routes from being advertised into the target area. Only the summary LSA appears in Area 0, which matches the observed behavior.

Why this answer

The `area 0 range 10.0.0.0 255.255.0.0` command on an OSPF ABR creates a single Type-3 summary LSA (10.0.0.0/16) for Area 0 and suppresses the advertisement of more-specific Type-3 LSAs (e.g., /24 routes) that fall within that range. This is the intended behavior of the `range` command: it summarizes routes at the ABR and prevents the individual component routes from being advertised into the backbone area. Since the host at 10.0.5.100 in Area 1 relies on those specific /24 routes to reach destinations in Area 0, the suppression breaks connectivity.

Exam trap

Cisco often tests the misconception that the `area range` command only summarizes routes without suppressing the more-specific LSAs, leading candidates to incorrectly attribute the connectivity loss to filtering or routing issues rather than the intended suppression behavior.

Why the other options are wrong

A

A distribute-list requires explicit configuration; its absence makes this an unsubstantiated guess.

B

Area border routers receive all LSAs from non-backbone areas; the area boundary does not stop LSA propagation to the ABR itself.

D

The null0 route is a loop-prevention mechanism, not an absolute traffic blocker; more-specific entries in the routing table take precedence.

145
MCQhard

A technician is troubleshooting an OSPF network. On a broadcast segment, R1 is the DR and R2 is the BDR. R1's interface GigabitEthernet0/0 is shut down for maintenance. The technician expects that R2 will assume the DR role, but instead a new DR election occurs and another router is elected DR. What is the most likely cause?

A.The OSPF hello and dead intervals on R2 do not match those of other routers on the segment.
B.R2 has an OSPF priority of 255.
C.The OSPF network type on the segment was changed to point-to-point.
D.R2 has an OSPF priority of 0.
AnswerD

On a broadcast OSPF network, a priority of 0 makes a router ineligible for DR or BDR election. Even though R2 was the BDR, its priority of 0 prevents it from taking over as DR when R1 fails. Consequently, a new DR election is triggered among the remaining eligible routers, and a router other than R2 becomes the new DR.

Why this answer

When the DR (R1) goes down, a new DR election occurs. R2 is the BDR, but if its OSPF priority is 0, it is ineligible to become the DR. Therefore, a new election takes place among the remaining routers with non-zero priorities, and another router wins the DR role.

Exam trap

Cisco often tests the misconception that the BDR automatically becomes DR when the DR fails, but the BDR only takes over if it has a non-zero priority; otherwise, a new election is triggered.

Why the other options are wrong

A

Neighbor adjacency failure due to timer mismatch would have prevented R2 from becoming BDR at all.

B

High priority increases the chance of being elected DR, not decrease it.

C

Point-to-point networks do not have DR/BDR elections, so R2 could not have been BDR.

146
MCQhard

R1 and R2 are directly connected via Ethernet on interface G0/0. Both interfaces are in the same subnet and configured for OSPF area 0. After enabling OSPF, R1's G0/0 is stuck in the INIT state in the OSPF neighbor table. What is the most likely cause?

A.MTU mismatch
B.Hello and dead timer mismatch
C.Missing router ID on both routers
D.A duplicate static route to 10.1.12.0/30
AnswerB

OSPF adjacency requires that Hello and Dead intervals match exactly between neighbors on the same link. The Dead interval is typically four times the Hello interval, but if one side is configured with different timers (e.g., 10/40 vs 30/120), the routers will discard each other's Hello packets and never reach the 2-Way state. This is the classic cause of a neighbor relationship that remains completely down, with no OSPF neighbor entry at all.

Why this answer

Being stuck in the INIT state means R1 has received Hello packets from R2 but has not transitioned to the 2-WAY state. This most often occurs when critical OSPF parameters like the hello and dead timers do not match. An MTU mismatch, while problematic, would cause the adjacency to fail during database exchange (Exstart/Exchange), not during initial neighbor formation.

Missing router IDs are not a real issue because routers auto-generate them, and a duplicate static route to the connected subnet would not affect OSPF neighbor discovery.

Exam trap

A common exam trap is selecting MTU mismatch or missing router IDs as the cause of OSPF adjacency failure. While MTU mismatches can cause adjacency issues during database exchange, they do not prevent the initial neighbor relationship from forming. Missing router IDs do not block adjacency because routers automatically generate IDs if none are configured.

Another trap is confusing static routes with neighbor discovery; static routes do not influence OSPF adjacency. The key is to focus on timer mismatches, as hello and dead intervals must be identical for routers to recognize each other as neighbors and establish adjacency.

Why the other options are wrong

A

MTU mismatch can cause OSPF adjacency problems during the database exchange phase, but it does not typically prevent the initial neighbor relationship from forming. Since the question states adjacency cannot form, MTU mismatch is unlikely the primary cause.

C

Missing router IDs do not prevent OSPF adjacency because routers automatically generate a router ID if none is configured. Therefore, this option is not a valid cause for adjacency failure.

D

A duplicate static route to 10.1.12.0/30 does not affect OSPF neighbor formation, as static routes are unrelated to OSPF adjacency processes. This option is irrelevant to the adjacency issue.

147
MCQhard

A network engineer is troubleshooting OSPFv3 adjacency between two directly connected Cisco routers, R1 and R2, both running IOS-XE. The engineer configures OSPFv3 on both routers but notices that the adjacency does not form. The engineer runs 'show ospfv3 neighbor' on R1 and sees no neighbors. What is the most likely cause of this issue?

A.The OSPFv3 process ID must match on both routers.
B.The interface GigabitEthernet0/0 is missing the 'ospfv3 1 ipv6 area 0' command.
C.The link-local addresses are not in the same subnet.
D.The router ID 1.1.1.1 is duplicated on R2.
AnswerB

With OSPFv3, enabling the OSPF process globally is not enough; each participating interface must be explicitly activated with the 'ospfv3 1 ipv6 area 0' interface-level configuration. This command assigns the interface to the OSPFv3 process 1 for address family IPv6 and places it into area 0. Without this command, the router will not send or listen for OSPFv3 Hello messages on GigabitEthernet0/0, so no neighbor adjacency can ever form. That is exactly why R2's neighbor entry is missing from the 'show ospfv3 neighbor' output.

Why this answer

OSPFv3 requires explicit interface-level configuration to enable the protocol on a specific interface. The correct command is 'ospfv3 1 ipv6 area 0' (or 'ipv6 ospf 1 area 0' for the traditional OSPFv3 configuration). Without this command, the interface does not participate in OSPFv3, so no Hello packets are sent or received, preventing adjacency formation.

Exam trap

Cisco often tests the distinction between OSPFv2 (where enabling the protocol under the routing process automatically activates it on all interfaces with 'network' statements) and OSPFv3 (which requires explicit per-interface activation), leading candidates to overlook the mandatory interface-level command.

Why the other options are wrong

A

This is a common misconception; OSPFv3 uses the router ID for neighbor identification, not the process ID.

C

Link-local addresses are automatically configured and do not affect OSPFv3 adjacency as long as they are unique.

D

In this scenario, no neighbors are seen, indicating a more fundamental issue like OSPFv3 not being enabled on the interface.

← PreviousPage 2 of 2 · 147 questions total

Ready to test yourself?

Try a timed practice session using only OSPF questions.