350-401 OSPF — 20 Questions

Question 1

A network engineer is troubleshooting OSPF adjacency issues between two routers connected via a Gigabit Ethernet link. The engineer notices that the routers are stuck in the EXSTART state. Both routers have the same MTU of 1500 bytes. What is the most likely cause of this issue?

Accepted Answer

One router has a lower IP MTU configured on the interface, causing the DBD packet to be dropped.. When OSPF routers are stuck in the EXSTART state, it typically indicates a problem with the Database Description (DBD) packet exchange. Even though both routers have the same configured MTU of 1500 bytes, one router may have a lower IP MTU on its interface (e.g., due to a different interface MTU or encapsulation overhead), causing the DBD packet to be fragmented or dropped. Since OSPF DBD packets are not fragmented, a mismatch in the actual IP MTU prevents the adjacency from progressing beyond EXSTART.

Answer

The OSPF network type is point-to-point on one router and broadcast on the other.

Answer

The OSPF hello and dead intervals are mismatched.

Answer

The OSPF router IDs are the same.

Question 2

An enterprise network uses OSPF as its IGP. The network engineer notices that a particular route learned via OSPF is not being installed in the routing table, even though the neighbor adjacency is up and the route appears in the OSPF database. The route is an external route redistributed from EIGRP. What is the most likely cause?

Accepted Answer

The forwarding address in the type 5 LSA is not reachable via an OSPF internal route.. Option C is correct because OSPF requires the forwarding address (FA) in a Type 5 LSA to be reachable via an OSPF internal route (intra-area or inter-area) for the external route to be installed in the routing table. If the FA is not reachable, the router will ignore the LSA and not install the route, even though the LSA exists in the OSPF database and the neighbor adjacency is up.

Answer

The OSPF process ID is different on the routers.

Answer

The external route has a higher administrative distance than the internal route.

Answer

The OSPF metric for the external route is too high.

Question 3

A network engineer is configuring OSPF in a multi-area design. The engineer wants to reduce the amount of LSA flooding and the size of the LSDB in area 0. Which OSPF feature should be implemented on the ABR to achieve this goal?

Accepted Answer

Configure the ABR with an area filter-list to filter type 3 LSAs.. Option B is correct because configuring an area filter-list on the ABR allows the engineer to filter Type 3 summary LSAs entering or leaving area 0. This directly reduces LSA flooding and shrinks the LSDB in area 0 by preventing specific inter-area prefixes from being advertised into the backbone, without altering the area type or requiring additional redistribution.

Answer

Configure area 0 as a stub area.

Answer

Configure OSPF database overflow protection.

Answer

Configure the ABR as an ASBR.

Question 4

A network engineer is troubleshooting an OSPF adjacency issue between two routers connected via a serial link. The adjacency is stuck in the INIT state. The engineer has verified that the IP addresses are in the same subnet and that the link is up. What is the most likely cause?

Accepted Answer

The OSPF hello interval is mismatched between the two routers.. The INIT state in OSPF indicates that a router has received a Hello packet from its neighbor but the neighbor has not yet seen its own Router ID in the received Hello. A mismatched Hello interval causes the routers to send Hellos at different rates, so one router may not receive a Hello within the expected Dead interval, preventing the neighbor from seeing its Router ID in the received Hello and thus stalling the adjacency in INIT.

Answer

The OSPF router IDs are the same.

Answer

The OSPF process ID is different.

Answer

The OSPF network type is point-to-point on one router and point-to-multipoint on the other.

Question 5

A network engineer is designing an OSPF network with multiple areas. The engineer wants to ensure that routers in area 2 can reach networks in area 0, but they should not learn any external routes from other ASs. Which OSPF area type should be configured for area 2?

Accepted Answer

Stub area. A stub area blocks Type 5 LSAs (external routes from other ASs) while allowing Type 3 summary LSAs from area 0. This ensures routers in area 2 can reach networks in area 0 via inter-area routes but do not learn external routes, meeting the requirement exactly.

Answer

Not-so-stubby area (NSSA)

Answer

Totally stubby area

Answer

Standard area

Question 6

A network engineer is troubleshooting an OSPF issue where a router is not learning a route to a network that is advertised via a type 5 LSA from an ASBR. The engineer checks the OSPF database and sees the type 5 LSA, but the route is not in the routing table. The forwarding address in the LSA is 0.0.0.0. What is the most likely cause?

Accepted Answer

The ASBR is not reachable via an OSPF internal route.. When a Type 5 LSA has a forwarding address of 0.0.0.0, OSPF routers will use the ASBR as the next hop for the external route. For the route to be installed in the routing table, the ASBR must be reachable via an OSPF intra-area or inter-area route. If the ASBR is not reachable (e.g., no valid OSPF route to the ASBR's router ID), the Type 5 LSA is considered unreachable and is not installed, even though it exists in the OSPF database.

Answer

The type 5 LSA has a metric of 16777215.

Answer

The OSPF process ID on the ASBR is different from the other routers.

Answer

The type 5 LSA is being filtered by an outbound route filter.

Question 7

A network engineer is configuring OSPF on a router that connects to two different ISPs. The engineer wants to prefer one ISP for all external routes unless that ISP's link fails, in which case the other ISP should be used. Which OSPF feature should be used to influence the path selection for external routes?

Accepted Answer

Use the 'distance ospf external' command to set a higher administrative distance for external routes from the less preferred ISP.. Option B is correct because the 'distance ospf external' command allows you to set a higher administrative distance for external OSPF routes learned from a specific neighbor or source, making the routes from the less preferred ISP less trustworthy. When the preferred ISP's link fails, those routes are removed, and the router will then use the external routes from the backup ISP due to their lower administrative distance (or default distance if not changed). This directly influences path selection for external routes without altering OSPF metrics or interface costs.

Answer

Configure the OSPF cost on the interface connecting to the preferred ISP to a lower value.

Answer

Use the 'max-metric' command on the router connecting to the less preferred ISP.

Answer

Configure the OSPF network type to point-to-point on both interfaces.

Question 8

A network engineer is troubleshooting an OSPF adjacency issue between two routers connected via a Frame Relay network. The adjacency is stuck in the 2WAY state. The engineer has verified that the routers are in the same area and have matching hello/dead intervals. What is the most likely cause?

Accepted Answer

The OSPF network type is non-broadcast and the neighbors are not manually configured.. In OSPF over Frame Relay, the default network type for physical interfaces is non-broadcast (NBMA). On an NBMA network, OSPF does not send multicast Hello packets; instead, neighbors must be manually configured using the 'neighbor' command. Without manual neighbor configuration, the routers will not proceed past the 2WAY state because they cannot form a full adjacency without explicit neighbor discovery.

Answer

The OSPF router priority is set to 0 on both routers.

Answer

The OSPF area ID is different.

Answer

The Frame Relay map is missing for the broadcast keyword.

Question 9

A network engineer is configuring OSPF on a router that has multiple interfaces in the same area. The engineer wants to ensure that the router does not become the designated router (DR) on any of these interfaces. What should the engineer do?

Accepted Answer

Set the OSPF priority to 0 on all interfaces.. Setting the OSPF priority to 0 on all interfaces prevents the router from participating in the DR/BDR election process. A router with priority 0 will never become the DR or BDR on any segment, regardless of its Router ID or other factors. This is the only method that guarantees the router will not be elected as DR on any interface.

Answer

Configure the OSPF network type as point-to-point on all interfaces.

Answer

Use the 'ip ospf dr-priority' command to set a high priority on other routers.

Answer

Configure the router as an ABR.

Question 10

A network engineer runs the following command on Router R1:

R1# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.2          1   FULL/DR         00:00:32    192.168.1.2     GigabitEthernet0/0
10.0.0.3          1   2WAY/DROTHER    00:00:35    192.168.1.3     GigabitEthernet0/0

Based on this output, what can be concluded?

Accepted Answer

R1 is a DROTHER on this segment.. The output shows R1 has a neighbor with state 2WAY/DROTHER (10.0.0.3), which indicates that R1 is also a DROTHER on this broadcast multiaccess segment. The FULL/DR neighbor (10.0.0.2) is the Designated Router, and since R1 is not the BDR (no FULL/BDR state), it must be a DROTHER.

Answer

R1 is the Backup Designated Router (BDR) on this segment.

Answer

R1 has a full OSPF adjacency with the neighbor 10.0.0.3.

Answer

The OSPF network type is point-to-point.

Question 11

A network engineer issues the following command on Router R2:

R2# show ip ospf interface GigabitEthernet0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.1.2/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 10
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2
  Backup Designated router (ID) 1.1.1.1, Interface address 192.168.1.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:03
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 2, Adjacent neighbor count is 2
    Adjacent with neighbor 1.1.1.1  (Backup Designated Router)
    Adjacent with neighbor 3.3.3.3

Based on this output, what can be concluded?

Accepted Answer

R2 has a full OSPF adjacency with all neighbors on this segment.. The output shows that R2 is the Designated Router (DR) on this broadcast segment, with two neighbors listed: 1.1.1.1 (BDR) and 3.3.3.3. The 'Adjacent neighbor count is 2' and both neighbors are listed as 'Adjacent with neighbor', confirming that R2 has formed full OSPF adjacencies with all neighbors on this segment. In OSPF broadcast networks, only the DR and BDR form full adjacencies with all routers, while non-DR/BDR routers only form full adjacencies with the DR and BDR.

Answer

R2 is the Backup Designated Router on this segment.

Answer

The OSPF cost to reach the network 192.168.1.0/24 is 20.

Answer

R2 will send hello packets every 40 seconds.

Question 12

A network engineer runs the following command on Router R3:

R3# show ip route ospf

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
O IA     10.1.1.0/24 [110/20] via 192.168.1.1, 00:12:34, GigabitEthernet0/0
O        10.2.2.0/24 [110/10] via 192.168.1.2, 00:15:22, GigabitEthernet0/0
O E2     10.3.3.0/24 [110/20] via 192.168.1.3, 00:08:11, GigabitEthernet0/0

Based on this output, what can be concluded?

Accepted Answer

The route to 10.3.3.0/24 is an external route redistributed into OSPF.. The route to 10.3.3.0/24 is marked as 'O E2' in the output, which stands for OSPF external type 2. This indicates that the route was redistributed into OSPF from another routing protocol or a different OSPF process, making it an external route. The 'E2' designation confirms it is an external route with a fixed metric that does not include the internal cost to the ASBR.

Answer

The route to 10.1.1.0/24 is in the same OSPF area as R3.

Answer

The metric for 10.3.3.0/24 includes the internal cost to the ASBR.

Answer

R3 is an ASBR.

Question 13

A network engineer issues the following command on Router R4:

R4# show ip ospf database

OSPF Router with ID (4.4.4.4) (Process ID 1)

Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         123         0x80000002 0x00A1B2 2
2.2.2.2         2.2.2.2         456         0x80000003 0x00B2C3 3
4.4.4.4         4.4.4.4         789         0x80000001 0x00C3D4 1

Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.2     2.2.2.2         234         0x80000001 0x00D4E5

Based on this output, what can be concluded?

Accepted Answer

Router 2.2.2.2 is the Designated Router on the segment 192.168.1.0/24.. The Net Link States entry shows the link ID 192.168.1.2 (the DR's interface IP) and ADV Router 2.2.2.2, which indicates that router 2.2.2.2 is the Designated Router (DR) on the 192.168.1.0/24 segment. In OSPF, only the DR originates the Type 2 (Network LSA) for a broadcast segment, so the advertising router in the Net Link States is always the DR.

Answer

Router 1.1.1.1 has three OSPF-enabled interfaces.

Answer

Router 4.4.4.4 is the BDR on the segment.

Answer

There are multiple broadcast segments in area 0.

Question 14

A network engineer runs the following command on Router R5:

R5# show ip ospf border-routers

OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 1.1.1.1 [110/10] via 192.168.1.1, GigabitEthernet0/0, ABR, Area 0, SPF 5
i 2.2.2.2 [110/20] via 192.168.1.2, GigabitEthernet0/0, ASBR, Area 0, SPF 5

Based on this output, what can be concluded?

Accepted Answer

Router 1.1.1.1 connects area 0 to another OSPF area.. The output shows an entry for 1.1.1.1 with the label 'ABR' (Area Border Router) and a route type of 'i' (intra-area). An ABR connects two or more OSPF areas, so Router 1.1.1.1 must be connecting Area 0 to another OSPF area. The metric [110/10] and next-hop 192.168.1.1 confirm it is reachable within Area 0.

Answer

Router 2.2.2.2 is an Area Border Router.

Answer

Router 1.1.1.1 is redistributing external routes into OSPF.

Answer

The route to 2.2.2.2 is an inter-area route.

Question 15

A network engineer issues the following command on Router R6:

R6# debug ip ospf hello

OSPF: Send hello to 224.0.0.5 via GigabitEthernet0/0 (192.168.1.6)
OSPF: Rcv hello from 1.1.1.1, GigabitEthernet0/0, area 0.0.0.0
  Neighbor state is 2WAY, options 0x2
OSPF: End of hello processing

Based on this output, what can be concluded?

Accepted Answer

R6 and 1.1.1.1 are neighbors, but a full adjacency may not yet be formed.. The debug output shows the neighbor state is 2WAY, which indicates that R6 has received a hello from 1.1.1.1 and bidirectional communication is established, but a full adjacency has not yet been formed. In OSPF, the 2WAY state is a prerequisite for advancing to the ExStart state and eventually to FULL, but on multiaccess networks, the router must also wait for the Designated Router (DR) and Backup Designated Router (BDR) election process to complete before proceeding. Therefore, option C correctly states that R6 and 1.1.1.1 are neighbors, but a full adjacency may not yet be formed.

Answer

R6 has formed a full adjacency with neighbor 1.1.1.1.

Answer

The hello packet was sent to the DR/BDR multicast address 224.0.0.6.

Answer

The OSPF network type is point-to-point.

Question 16

A network engineer runs the following command on Router R7:

R7# show ip ospf virtual-links

Virtual Link OSPF_VL0 to router 2.2.2.2 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface GigabitEthernet0/1, Cost of using 10
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:07
  Adjacency State FULL

Based on this output, what can be concluded?

Accepted Answer

Router 2.2.2.2 is the other endpoint of the virtual link.. The output shows that the virtual link OSPF_VL0 to router 2.2.2.2 is up and the adjacency state is FULL. This confirms that router 2.2.2.2 is the remote endpoint of the virtual link, as the command displays the router ID of the neighbor at the other end of the virtual link.

Answer

The virtual link is used to connect area 0 to a non-backbone area.

Answer

The virtual link is down.

Answer

The virtual link uses area 0 as the transit area.

Question 17

A network engineer issues the following command on Router R8:

R8# show ip ospf neighbor detail

Neighbor 1.1.1.1, interface address 192.168.1.1
    In the area 0 via interface GigabitEthernet0/0
    Neighbor priority is 1, State is FULL, 6 state changes
    DR is 192.168.1.2, BDR is 192.168.1.1
    Options is 0x42 (L LSR LSRR L LSR)
    Dead timer due in 00:00:34
    Neighbor is up for 00:12:45
    Index 1/1/1, retransmission queue length 0, number of retransmission 0
    First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)
    Last retransmission scan length is 0, last retransmission scan time is 0 msec

Based on this output, what can be concluded?

Accepted Answer

Router 1.1.1.1 is the Backup Designated Router.. The output shows 'BDR is 192.168.1.1', which is the interface address of neighbor 1.1.1.1. This directly indicates that router 1.1.1.1 is the Backup Designated Router on this segment. The neighbor state is FULL, confirming adjacency is fully established.

Answer

Router 1.1.1.1 is the Designated Router on this segment.

Answer

The neighbor state is 2WAY.

Answer

The dead timer is 40 seconds.

Question 18

A network engineer runs the following command on Router R9:

R9# show ip ospf interface brief

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/0        1     0               192.168.1.9/24     10    DR    2/2
Gi0/1        1     1               10.0.0.9/24        20    BDR   1/1
Lo0          1     0               9.9.9.9/32         1     LOOP  0/0

Based on this output, what can be concluded?

Accepted Answer

R9 has two fully adjacent neighbors on Gi0/0.. The output shows that on interface Gi0/0, R9 has a state of DR (Designated Router) and 2 fully adjacent neighbors (Nbrs F/C = 2/2). The '2/2' indicates 2 neighbors in a full state out of 2 total neighbors, meaning both neighbors have completed the OSPF adjacency process. Therefore, R9 has two fully adjacent neighbors on Gi0/0, making option B correct.

Answer

R9 is the Designated Router on the segment connected to Gi0/1.

Answer

The loopback interface Lo0 is advertised as a /24 network.

Answer

R9 is an Area Border Router.

Question 19

Router R1 has the following OSPF configuration:

interface GigabitEthernet0/0
 ip address 10.1.1.1 255.255.255.0
 ip ospf 1 area 0
 ip ospf network point-to-point
!
router ospf 1
 router-id 1.1.1.1
 network 10.0.0.0 0.255.255.255 area 0

What is the effect of the 'ip ospf network point-to-point' command on this interface?

Accepted Answer

It changes the OSPF network type to point-to-point, eliminating DR/BDR election and reducing hello timer to 10 seconds.. The 'ip ospf network point-to-point' command changes the OSPF network type on the interface from the default (broadcast for Ethernet) to point-to-point. This eliminates the need for a Designated Router (DR) and Backup Designated Router (BDR) election, as point-to-point links have only two neighbors. Additionally, the OSPF hello timer on a point-to-point network defaults to 10 seconds (versus 30 seconds for non-broadcast), and the dead timer is 40 seconds.

Answer

It disables OSPF on the interface.

Answer

It enables OSPF authentication on the interface.

Answer

It sets the OSPF cost to 1.

Question 20

Consider the following OSPF configuration on router R2:

interface GigabitEthernet0/0
 ip address 192.168.1.2 255.255.255.0
 ip ospf 1 area 0
 ip ospf hello-interval 5
!
router ospf 1
 router-id 2.2.2.2
 network 192.168.1.0 0.0.0.255 area 0

Which statement is true about this configuration?

Accepted Answer

The OSPF dead interval is automatically set to 20 seconds.. The correct answer is A because the OSPF dead interval is automatically set to four times the hello interval when the hello interval is manually configured. By default, OSPF uses a hello interval of 10 seconds and a dead interval of 40 seconds on broadcast networks. However, when you explicitly set the hello interval to 5 seconds using the 'ip ospf hello-interval 5' command, the router automatically adjusts the dead interval to 20 seconds (4 × 5 seconds) to maintain the standard ratio, unless the dead interval is also manually configured.

Answer

The OSPF dead interval remains at the default of 40 seconds.

Answer

This configuration will cause OSPF to use MD5 authentication.

Answer

The OSPF network type changes to point-to-point.