CCNA Eigrp Questions

MCQhard

A router receives a destination prefix from EIGRP with administrative distance 90 and also from OSPF with administrative distance 110. The prefix length is identical. Which route source is preferred?

A.EIGRP

B.OSPF

C.Both equally

D.Neither, because route sources cannot overlap

AnswerA

This is correct because EIGRP’s default administrative distance of 90 is lower than OSPF’s 110.

Why this answer

The EIGRP route is preferred because its administrative distance is lower. In practical terms, once the prefix length is the same, the router compares the trustworthiness of the route source. Lower administrative distance wins. Since 90 is lower than 110, EIGRP is preferred over OSPF for that destination.

This is an administrative-distance comparison question, not a longest-prefix question. The key is that the prefix length is equal, so source preference becomes the deciding factor.

Exam trap

A frequent exam trap is to assume that OSPF routes might be preferred over EIGRP routes simply because OSPF is a widely used IGP or because of metric comparisons within OSPF. Candidates often confuse administrative distance with routing metrics, mistakenly thinking that the lower metric route is preferred regardless of protocol. However, administrative distance is the first criterion when routes come from different protocols.

Another trap is to think that routes from different protocols cannot overlap or that prefix length alone determines preference. The key is that when prefix lengths are equal, the router uses administrative distance to select the best route, so EIGRP’s lower AD of 90 always beats OSPF’s 110.

Why the other options are wrong

This option is incorrect because OSPF’s administrative distance of 110 is higher than EIGRP’s 90, so OSPF routes are less preferred when both advertise the same prefix length.

This option is incorrect because routers do not treat routes from different protocols with equal administrative distance; they prefer the route with the lower administrative distance, so both routes are not equally preferred.

This option is incorrect because routers can and often do receive overlapping routes from multiple routing protocols and must compare administrative distance to choose the best route.

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Multi-Selectmedium

Which three of the following statements about distance vector routing protocols (e.g., RIP, EIGRP) are correct? (Choose all that apply. There are three correct answers.)

Select 4 answers

.They rely on the Bellman-Ford algorithm for route computation.

.RIPv2 uses multicast address 224.0.0.9 to send routing updates.

.EIGRP maintains separate feasible distance and advertised distance values for each route.

.They require a full topological map of the network to calculate the best path.

.EIGRP supports both equal-cost and unequal-cost load balancing by default.

.Split horizon prevents routing loops by not advertising a route back out the interface from which it was learned.

Why this answer

RIPv2 sends updates to the multicast address 224.0.0.9, ensuring only RIPv2 routers process them. EIGRP maintains both feasible distance (best metric to a destination) and advertised distance (metric reported by the next-hop router) to support its loop-free DUAL algorithm. Split horizon prevents routing loops by not advertising a route back out the interface from which it was learned.

The statement about reliance on Bellman‑Ford is false because EIGRP uses DUAL, not Bellman‑Ford; the other two wrong options confuse distance vector behavior with link-state (full topology map) or assume unequal-cost load balancing is default when it requires the variance command.

Exam trap

Cisco often tests the distinction between default behavior and optional features, so the trap here is assuming that EIGRP's unequal-cost load balancing is enabled by default when it actually requires the 'variance' command to activate.

Why the other options are wrong

EIGRP, a distance vector protocol listed as an example, uses DUAL, not Bellman-Ford, making the statement inaccurate.

Requiring a full topological map is characteristic of link-state protocols, not distance vector protocols.

EIGRP does not support unequal-cost load balancing by default; it must be explicitly enabled with the 'variance' command.

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PBQhard

You are connected to R1. Configure R1 so that it uses a floating static route to reach the 203.0.113.0/24 network via R2 only when the primary route (learned via EIGRP) fails. The primary route has an administrative distance of 90. Currently, R1 has no route to 203.0.113.0/24 because EIGRP is down on the direct link. Ensure the floating static route is installed and used.

Network Topology

Hints

•The primary route would be learned via EIGRP with AD 90.
•A floating static route must have an AD higher than 90 to be less preferred.
•Use the 'ip route' command with an AD value between 91 and 255.

A.ip route 203.0.113.0 255.255.255.0 10.0.0.2 95

B.ip route 203.0.113.0 255.255.255.0 10.0.0.2 90

C.ip route 203.0.113.0 255.255.255.0 10.0.0.2 85

D.ip route 203.0.113.0 255.255.255.0 10.0.0.2

AnswerA

solution

! R1
ip route 203.0.113.0 255.255.255.0 10.0.0.2 95

Why this answer

The issue is that R1 has no route to 203.0.113.0/24 because EIGRP is not working (likely due to misconfiguration or link failure). A floating static route with an administrative distance greater than EIGRP's default AD of 90 is needed. By configuring a static route to 203.0.113.0/24 via next-hop 10.0.0.2 with AD 95, the static route will be used only when EIGRP is down (since 95 > 90, EIGRP is preferred when active).

The command 'ip route 203.0.113.0 255.255.255.0 10.0.0.2 95' accomplishes this.

Exam trap

Remember that floating static routes require an administrative distance higher than the primary route's AD. Do not use the default AD (1) or match the primary AD; always set a higher value.

Why the other options are wrong

The AD must be greater than 90 to ensure the static route is only used when EIGRP fails. An AD of 90 does not create a floating static route.

A floating static route must have a higher AD than the primary route. An AD of 85 is lower than 90, so it would be preferred over EIGRP.

The default AD for static routes is 1, which is lower than EIGRP's 90. Without specifying a higher AD, the static route will be preferred and not act as a floating static route.

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PBQmedium

You are connected to R1 via console. R1 and R2 are configured with EIGRP AS 100. R1's loopback0 (1.1.1.1/32) should be advertised into EIGRP. However, after configuration, R2 does not have a route to 1.1.1.1/32. You need to verify the EIGRP configuration on R1 and R2 to determine why the route is missing. Use show commands to identify the issue.

Network Topology

Hints

•Check if the loopback interface is configured as passive or if there is a network statement issue.
•The 'passive-interface default' command makes all interfaces passive unless explicitly configured otherwise.
•Ensure that the network statement for the loopback includes the correct wildcard mask.

A.Use 'show ip eigrp interfaces' on R1 to verify that EIGRP is enabled on the interface facing R2 and that it is not passive.

B.Use 'show ip eigrp topology' on R1 to confirm that the 1.1.1.1/32 route is in the EIGRP topology table.

C.Use 'show ip route eigrp' on R1 to verify that the 1.1.1.1/32 route is in the routing table.

D.Use 'show ip protocols' on R2 to verify that EIGRP AS 100 is configured and that the network statement includes the subnet of the interface facing R1.

AnswerA

solution

! R1
show ip eigrp neighbors
show ip eigrp topology
show ip route eigrp
show running-config | section router eigrp

! R2
show ip route eigrp

Why this answer

The root cause is that R1's interface facing R2 is passive, preventing EIGRP neighbor adjacency. 'show ip eigrp interfaces' verifies the passive state, identifying why no routes are exchanged. Option B is incorrect because the local topology table may contain the route, but without an active neighbor, it will not be advertised; the topology check alone is insufficient. Option C is incorrect because the routing table may also show the locally connected route, but that does not explain why R2 lacks it.

Option D is incorrect because checking R2's EIGRP configuration does not reveal R1's passive interface, which is the actual problem.

Exam trap

Be careful: 'passive-interface default' makes all interfaces passive, including the one needed for neighbor adjacency. You must use 'no passive-interface' on the specific interface to allow EIGRP to form a neighbor. Also, remember that passive interfaces can still advertise routes, but they do not form adjacencies.

Why the other options are wrong

The route being in R1's topology does not guarantee it is advertised to R2; the problem is more likely with the neighbor adjacency or outbound filters.

The loopback is a directly connected interface, so it will not appear in the EIGRP routing table; it is injected into EIGRP via the network statement. This command is irrelevant for checking the advertisement.

The problem is likely on R1, not R2. R2's configuration may be fine, but if R1 is not sending the route, R2 will never learn it. This command focuses on R2, which is not the source of the issue.

MCQmedium

A router learns 10.10.10.0/24 from OSPF and EIGRP at the same time. OSPF reports a metric of 20, and EIGRP reports a metric of 30720. Which route is installed in the routing table by default?

A.The OSPF route, because 20 is lower than 30720

B.The EIGRP route, because its administrative distance is lower

C.Both routes, because they point to the same prefix

D.Neither route, because the metrics are not comparable

AnswerB

Correct. EIGRP wins because its default administrative distance is lower than OSPF.

Why this answer

When the same prefix is learned from different routing protocols, the router compares administrative distance first. EIGRP internal routes use AD 90, while OSPF uses AD 110, so the EIGRP route is preferred.

Exam trap

Remember, administrative distance is the first criterion for route selection between different protocols, not the metric.

Why the other options are wrong

Metrics from different routing protocols are not comparable because each protocol uses its own metric calculation (e.g., OSPF uses cost based on bandwidth, EIGRP uses composite metric based on bandwidth and delay). The router uses administrative distance to choose between routes from different protocols, not metric values.

A router installs only the best route to a destination in the routing table, unless equal-cost load balancing is configured. Since OSPF and EIGRP have different administrative distances, only the route with the lower AD is installed. Both routes cannot be installed simultaneously for the same prefix.

The router does compare routes from different protocols using administrative distance, not metrics. Since the metrics are from different protocols, they are not directly comparable, but the router still selects one route based on AD. Therefore, one route will be installed.

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MCQhard

A router learns the same destination prefix from OSPF and EIGRP. The prefix length is identical, and both routes are valid. Which route is preferred by default?

A.The EIGRP route

B.The OSPF route

C.Both routes are installed equally because the prefix length matches

D.Neither route is used because protocols cannot advertise the same prefix

AnswerA

This is correct because EIGRP's default administrative distance is lower than OSPF's.

Why this answer

The EIGRP route is preferred by default because EIGRP has a lower default administrative distance than OSPF. In practical terms, once the prefix length is the same, the router compares source trust. Lower administrative distance wins. EIGRP’s default of 90 beats OSPF’s default of 110.

This is not a longest-prefix question. The prefix is identical, so the decision is about source preference rather than specificity.

Exam trap

A frequent exam trap is believing that when two routing protocols advertise the same prefix with identical prefix lengths, the router installs both routes equally or performs load balancing. This misconception ignores the role of administrative distance, which is the primary factor in route preference when prefix lengths match. Another trap is thinking that OSPF is always preferred because it is a widely used IGP, but Cisco routers prioritize routes based on AD values, not protocol popularity.

Misunderstanding this can lead to incorrect answers about route selection in multi-protocol environments.

Why the other options are wrong

This option is incorrect because OSPF’s default administrative distance (110) is higher than EIGRP’s (90), making OSPF routes less preferred when both advertise the same prefix.

This option is incorrect because equal prefix length does not cause routers to install both routes equally; administrative distance determines which route is preferred and installed.

This option is incorrect because routers can receive and compare the same prefix from multiple routing protocols; they do not reject prefixes simply because they come from different sources.

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PBQhard

You are connected to R1 via the console. R1 is a Cisco ISR 4321 router running IOS-XE. The network team has recently changed the routing protocol from EIGRP to OSPF, but some routes are missing from the routing table. You need to analyze the OSPF neighbor states and LSDB to identify the issue.

Network Topology

Hints

•Check if OSPF neighbors are in FULL state.
•Look for mismatched OSPF network types or hello intervals.
•Verify that both routers are in area 0.

A.Use 'show ip ospf neighbor' to check neighbor state; if not FULL, examine 'show ip ospf interface' for mismatched hello/dead intervals or network type.

B.Use 'show ip route ospf' to verify OSPF routes; if missing, re-enter the OSPF process and redistribute connected routes.

C.Use 'debug ip ospf events' to monitor OSPF packets; if no packets are seen, reconfigure OSPF router ID and clear the OSPF process.

D.Use 'show ip protocols' to verify OSPF process configuration; if incorrect, delete and recreate the OSPF process with the correct network statements.

AnswerA

solution

! R1
show ip ospf neighbor
show ip ospf interface gigabitethernet0/0
show ip route ospf

! R2
show ip ospf neighbor
show ip ospf interface

Why this answer

OSPF requires consistent hello/dead intervals and network types to form adjacency. If the neighbor is not in FULL state, routes will not be exchanged. Checking 'show ip ospf neighbor' reveals the state; mismatched parameters can be corrected by adjusting interface configuration.

Exam trap

The exam trap is that candidates may jump to reconfiguring the OSPF process or redistribution when routes are missing, but the root cause is often a simple mismatch in OSPF interface parameters. Always check neighbor states first.

Why the other options are wrong

The specific factual error is that 'show ip route ospf' shows only existing OSPF routes, not neighbor states. Redistribution is not needed for directly connected interfaces in the same OSPF area.

The specific factual error is that debugging is not the initial diagnostic step; it should be used after verifying neighbor states and interface parameters. Changing the router ID is unnecessary unless there is a duplicate router ID issue.

The specific factual error is that 'show ip protocols' does not display per-interface OSPF parameters. The problem is likely at the interface level, not the process level.

MCQmedium

A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?

A.The OSPF route because OSPF is link-state

B.The EIGRP route because it has the lower administrative distance

C.The route with the lower metric value regardless of protocol

D.Both routes are always installed

AnswerB

Correct. Lower AD wins between different routing protocols.

Why this answer

When identical prefixes are learned from different routing protocols, the router compares administrative distance first. EIGRP internal routes use AD 90, while OSPF uses AD 110.

Exam trap

Remember that lower administrative distance values indicate higher preference. Don't confuse protocol complexity with route preference.

Why the other options are wrong

The type of routing protocol (link-state vs. distance vector) does not determine route preference when comparing routes from different protocols. Route selection is based on administrative distance, not protocol characteristics.

Metrics are only comparable within the same routing protocol. Different protocols use different metrics (e.g., OSPF uses cost, EIGRP uses composite metric), so they cannot be directly compared. Administrative distance is used to choose between protocols.

By default, a router installs only the best route (lowest AD) for a given prefix into the routing table. Both routes are not installed unless features like equal-cost multipath or policy routing are configured, which is not the case here.

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PBQmedium

You are connected to R1 via the console. R1 and R2 are configured with EIGRP AS 100. R1 has two paths to the 172.16.1.0/24 network: one via a FastEthernet link to R2 (bandwidth 100 Mbps, delay 100 microseconds) and another via a serial link to R2 (bandwidth 1.544 Mbps, delay 20000 microseconds). The EIGRP metric is calculated using the default K-values. The FastEthernet link is preferred, but you need to make the serial link the backup by adjusting the administrative distance.

Network Topology

Hints

•The default administrative distance for EIGRP is 90 for internal routes.
•Use an administrative distance value greater than 90 to make the static route less preferred.
•The command uses 'ip route' with an AD value at the end.

A.Configure a static route to 172.16.1.0/24 with next-hop 10.0.0.6 and administrative distance 170.

B.Configure a static route to 172.16.1.0/24 with next-hop 10.0.0.6 and administrative distance 90.

C.Configure a static route to 172.16.1.0/24 with next-hop 10.0.0.6 and administrative distance 1.

D.Configure a static route to 172.16.1.0/24 with next-hop 10.0.0.6 and administrative distance 255.

AnswerA

solution

! R1
ip route 172.16.1.0 255.255.255.0 10.0.0.6 170

Why this answer

A floating static route is configured with an administrative distance of 170, which is higher than EIGRP's default distance of 90. This ensures that the static route is only used when the EIGRP route is not available. The static route points to the next-hop IP address of R2's serial interface.

Exam trap

Remember that a floating static route must have an administrative distance higher than the dynamic protocol's AD. EIGRP's default AD is 90, so choose an AD like 170. Avoid using AD 255, as it means the route is not installed.

Why the other options are wrong

The specific factual error is that an AD of 90 does not make the static route less preferred than EIGRP; it creates equal preference.

The specific factual error is that a lower AD makes the route more preferred, so the static route would override EIGRP.

The specific factual error is that AD 255 means the route is not trusted and is effectively ignored.

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

This is correct because EIGRP is preferred over OSPF by default due to administrative distance.

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

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.

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.

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.

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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

Since the routes have different prefix lengths, they are treated as separate destinations and both are installed.

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

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

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

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

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PBQhard

You are connected to R1 via the console. The network currently uses EIGRP as its IGP, but you recently configured a static default route toward R2 (next-hop 203.0.113.2) to reach the Internet. However, traffic from R1 to the Internet is not taking the expected path. Examine the provided routing table and partial configuration, then fix the issue so that the static default route is used only when the EIGRP-learned default route is unavailable.

Network Topology

Hints

•Compare the administrative distances of the two default routes in the routing table.
•The static route currently has a lower AD (1) than EIGRP (90), so it is not acting as a backup.
•To make a static route a floating static, you need to configure an AD higher than the dynamic protocol's AD.

A.Configure the static route with an administrative distance of 100 using 'ip route 0.0.0.0 0.0.0.0 203.0.113.2 100'.

B.Remove the EIGRP default route by configuring 'no network 0.0.0.0' under the EIGRP process.

C.Increase the metric of the EIGRP default route to make it less preferred than the static route.

D.Configure the static route with a next-hop of 203.0.113.2 and a metric of 100.

AnswerA

solution

! R1
configure terminal
no ip route 0.0.0.0 0.0.0.0 203.0.113.2 10
ip route 0.0.0.0 0.0.0.0 203.0.113.2 100
end
write memory

Why this answer

The static default route was configured with an administrative distance (AD) of 10, which is lower than the EIGRP‑learned default route's AD of 90. This made the static route the preferred path, overriding the intended primary EIGRP route. To create a floating static route that only activates when the EIGRP route disappears, the static AD must be raised above 90—using 'ip route 0.0.0.0 0.0.0.0 203.0.113.2 100' ensures the EIGRP route is primary.

Option B is wrong because removing the EIGRP route eliminates the preferred path entirely, defeating the backup purpose. Option C fails because EIGRP metrics are irrelevant against a lower‑AD static route; AD dictates route source preference. Option D is incorrect because static routes do not accept a metric parameter—the trailing number sets the AD, not a metric.

Exam trap

Don't confuse administrative distance with metric. When comparing routes from different sources (static vs. EIGRP), AD is the deciding factor.

Also, remember that static routes use AD, not metric, to influence preference. The 'ip route' command syntax does not include a metric parameter.

Why the other options are wrong

The specific factual error: EIGRP does not use 'network 0.0.0.0' to advertise a default route; default routes are typically redistributed or generated via 'ip default-network' or redistribution. Removing the EIGRP default route would break the intended primary path.

The specific factual error: Administrative distance is the primary factor for route selection between different routing protocols or sources. Changing the EIGRP metric does not affect the comparison with a static route.

The specific factual error: The 'ip route' command syntax is 'ip route prefix mask {next-hop | interface} [distance] [name] [permanent] [tag tag]'. There is no metric parameter. The correct way to make a static route less preferred is to set a higher administrative distance.

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Multi-Selectmedium

Which three of the following are valid features of Enhanced Interior Gateway Routing Protocol (EIGRP)? (Choose three.)

Select 3 answers

.EIGRP maintains a topology table containing all routes learned from neighbors.

.EIGRP uses the Diffusing Update Algorithm (DUAL) to ensure loop-free paths.

.EIGRP supports unequal-cost load balancing using the 'variance' command.

.EIGRP is a link-state routing protocol similar to OSPF.

.EIGRP uses multicast address 224.0.0.5 for all neighbor communications.

.EIGRP automatically summarizes routes at classful boundaries by default on all interfaces.

Why this answer

All three statements are correct features of EIGRP. EIGRP maintains a topology table that stores all routes learned from directly connected neighbors, including feasible successors. It uses the Diffusing Update Algorithm (DUAL) to guarantee loop-free paths by performing a diffusing computation when a route is lost.

Additionally, EIGRP supports unequal-cost load balancing by using the 'variance' command, which allows traffic to be distributed across multiple paths with different metrics, as long as the metric of the alternate path is within the variance multiplier times the best metric.

Exam trap

Cisco often tests the distinction between EIGRP's topology table (which stores all learned routes) and its routing table (which stores only the best routes), and candidates may confuse the 'variance' command with equal-cost load balancing only, forgetting that it enables unequal-cost load balancing.

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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

This is correct because the destination falls inside the more specific /28 prefix.

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

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.

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.

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.

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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

Different protocols are compared by AD first.

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

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.

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.

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.

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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

This is correct because both routes are /24, so administrative distance becomes decisive and EIGRP wins.

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

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

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

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

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