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OSPF Troubleshooting Scenarios

Practise 300-410 Cisco CCNP ENARSI 300-410 OSPF questions covering neighbour states, router IDs, areas, timers, passive interfaces, OSPF cost, route selection, and command-output troubleshooting.

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300-410

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Scenario guide

How to approach ospf troubleshooting scenarios

OSPF neighbour adjacencies, route advertisements, and DR/BDR elections appear consistently on the CCNA. These questions test whether you can read OSPF state from show commands and identify why two routers fail to reach FULL adjacency or why a route isn't being learned.

Quick answer

OSPF questions usually test neighbour formation, areas, router IDs, route preference, metrics and command-output interpretation.

How OSPF neighbours form and why adjacency can fail.

How router ID, area ID, timers, passive interfaces and authentication affect OSPF.

How OSPF cost influences route selection.

How to read show ip route and show ip ospf neighbor output.

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Practice set

Practice scenarios

Question 1mediumdrag order

Review the full OSPF breakdown →

Drag and drop the steps to troubleshoot OSPF DR/BDR election on a multi-access segment into the correct order, from first to last.

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

Question 2hardmultiple choice

Review the full OSPF breakdown →

An engineer configures mutual redistribution between OSPF and EIGRP on a router. Both protocols have routes for the same prefix. The engineer also applies an inbound ACL on the OSPF interface to deny certain routes from being learned via OSPF. After the ACL is applied, the router still has the prefix in the routing table, but it is learned via EIGRP instead of OSPF. What is the most likely explanation?

A
The ACL blocks OSPF updates, so the router learns the prefix via EIGRP with a lower AD.
The ACL prevents OSPF from learning the route, but redistribution allows EIGRP to carry it, and EIGRP's lower AD makes it the best path.
B
The ACL is applied outbound, so it filters OSPF routes being sent to the neighbor.
Why wrong: Outbound ACL would affect outgoing updates, not incoming learning.
C
The router has a static route for the prefix with AD 1.
Why wrong: Static route would override both, but the question does not mention static.
D
The OSPF process has a distribute-list that is filtering the route.
Why wrong: Distribute-list would filter the route from the routing table, but the ACL is the configured filter.

Full breakdown with real-world context →

Question 3hardmultiple choice

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In a VRF-Lite setup, Router R1 and R2 are running OSPF in VRF-A. R1 has interface Gig0/0 in VRF-A with ip ospf network point-to-point. R2 has interface Gig0/1 in VRF-A with default network type (broadcast). The link between them is a direct Ethernet connection. OSPF neighbors are not forming. What is the root cause?

A
The OSPF network types are mismatched: one side is point-to-point, the other is broadcast, causing neighbor adjacency failure.
Correct: OSPF requires matching network types on the same link; mismatch leads to no neighbor formation.
B
The VRF names must match for OSPF to form neighbors.
Why wrong: Incorrect: VRF names are local; OSPF process is per VRF, but network type mismatch is the issue.
C
The OSPF process ID must be the same on both routers.
Why wrong: Incorrect: OSPF process ID is local and does not need to match.
D
The interface must be configured with the same IP subnet mask.
Why wrong: Incorrect: While IP subnet mismatch can cause issues, the primary problem here is network type mismatch.

Full breakdown with real-world context →

Question 4hardmultiple choice

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An engineer redistributes OSPF routes into EIGRP. The OSPF routes have a metric of 20. After redistribution, the EIGRP topology table shows the routes but they are not installed in the routing table. The 'show ip eigrp topology' shows the route in active state. Which is the most likely explanation?

A
The 'default-metric' command was not configured, so the routes are redistributed with an infinite metric.
Why wrong: Infinite metric would show the route in passive state but not installed.
B
The EIGRP neighbor is stuck-in-active because a query was sent but no reply was received due to a unidirectional link.
Active state indicates a query is outstanding, often due to a unidirectional link.
C
The OSPF routes are external type 2, which are not redistributed by default.
Why wrong: OSPF external routes are redistributed by default.
D
The 'route-map' used for redistribution has a 'match ip address' that does not match the OSPF routes.
Why wrong: If route-map denies, the route would not appear in topology table.

Full breakdown with real-world context →

Question 5mediummultiple choice

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A network engineer is troubleshooting an OSPFv2 adjacency issue between two routers connected via a VLAN trunk. R1 and R2 are in different VLANs but are connected through a Layer 2 switch. The engineer has configured 'ip ospf 1 area 0' on the subinterfaces. The adjacency forms but is stuck in INIT state. What is the most likely cause?

A
The routers have the same OSPF router ID.
When a router receives a hello packet with its own router ID, it ignores it, causing the adjacency to stay in INIT.
B
The OSPF hello interval is mismatched between the two routers.
Why wrong: A hello interval mismatch would cause the adjacency to not form at all, not to be stuck in INIT.
C
The subinterfaces are in different VLANs, preventing Layer 2 communication.
Why wrong: If they were in different VLANs, the routers would not receive each other's hellos, so no adjacency would form.
D
The OSPF network type is set to point-to-point on one side and broadcast on the other.
Why wrong: A network type mismatch would cause the adjacency to be stuck in EXSTART or EXCHANGE, not INIT.

Full breakdown with real-world context →

Question 6hardmultiple choice

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A network engineer notices that routes redistributed from EIGRP into OSPF are causing routing loops. Router R1 has the following relevant configuration: router eigrp 100 redistribute ospf 1 metric 10000 100 255 1 1500 router ospf 1 redistribute eigrp 100 subnets metric-type 1 Router R2 shows: show ip route 192.168.1.0 Routing entry for 192.168.1.0/24 Known via "ospf 1", distance 110, metric 20, type E1 Last update from 10.1.1.1, 00:01:30 ago Also, R2 has a default route via R1. What is the root cause?

A
Mutual redistribution without filtering causes a routing loop; apply distribute-list or route-map with tags.
Without filtering, routes are redistributed back and forth, creating loops. Tags and filters break the cycle.
B
The metric-type 1 should be type 2 to avoid loops.
Why wrong: Metric type does not prevent loops; filtering does.
C
The EIGRP metric values are incorrect; they should match OSPF cost.
Why wrong: Metric conversion is not the root cause of loops.
D
The subnets keyword is missing in the redistribute command for OSPF.
Why wrong: The subnets keyword is present; missing it would cause classful redistribution, not loops.

Full breakdown with real-world context →

Question 7hardmultiple choice

Review the full OSPF breakdown →

A network engineer runs the following command on Router R1:

R1# show ip ospf neighbor detail

 Neighbor 10.1.1.4, interface address 192.168.14.4

In the area 0 via interface GigabitEthernet0/2

Neighbor priority is 1, State is 2WAY, 2 state changes

DR is 10.1.1.1, BDR is 10.1.1.4 Options is 0x12 (L L S R) Dead timer due in 00:00:38

Neighbor is up for 00:05:22

Index 1/1/1, retransmission queue length 0, number of retransmission 0 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 0, time is 0 msec Last retransmission scan time is 0 msec

Based on this output, what is the problem?

A
The neighbor is in 2WAY state, meaning the adjacency is not fully established.
2WAY state indicates that the routers have seen each other's hello packets but have not exchanged LSAs. FULL is required for complete adjacency.
B
Router R1 is the DR and the neighbor is the BDR, so the state is correct.
Why wrong: Even if R1 is DR and neighbor is BDR, they should still form a FULL adjacency, not 2WAY.
C
The dead timer is about to expire, causing the neighbor to go down.
Why wrong: Dead timer is 00:00:38, which is not near zero.
D
The neighbor has a lower priority than R1, so it remains in 2WAY.
Why wrong: Priority does not affect the state progression beyond 2WAY; all routers should go to FULL with DR/BDR.

Full breakdown with real-world context →

Question 8mediummultiple choice

Review the full OSPF breakdown →

A network engineer runs the following command on Router R1:

R1# show ip ospf neighbor vrf BLUE

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.2          1   FULL/DR        00:00:32    10.1.1.2        GigabitEthernet0/0
10.0.0.3          1   2WAY/DROTHER   00:00:35    10.1.2.2        GigabitEthernet0/1

Based on this output, which statement is correct?

A
Neighbor 10.0.0.3 is not forming a full adjacency because it is a DROTHER.
Why wrong: DROTHERs remain in 2WAY state and do not need to become FULL; this is normal.
B
Neighbor 10.0.0.2 is the Backup Designated Router.
Why wrong: The state shows FULL/DR, meaning it is the Designated Router.
C
Both neighbors are in the FULL state.
Why wrong: Neighbor 10.0.0.3 is in 2WAY state.
D
The OSPF adjacencies are functioning normally.
The states are appropriate for the network type; FULL for DR and 2WAY for DROTHER.

Full breakdown with real-world context →

Question 9hardmultiple choice

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R1 and R2 are OSPF neighbors over a tunnel interface with BFD enabled. R1#show ip ospf interface tunnel0 shows 'BFD is enabled' but R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Down'. The tunnel is up and OSPF adjacency is full. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under tunnel0. R2 has same. What is the root cause?

A
BFD is not supported over tunnel interfaces; it requires physical interfaces.
BFD cannot be used over tunnel interfaces because BFD requires direct layer 2 connectivity; tunnels are layer 3 virtual interfaces.
B
The tunnel mode must be changed to 'ipsec' for BFD to work.
Why wrong: IPsec does not enable BFD over tunnels.
C
OSPF must be configured with 'bfd all-interfaces' to work over tunnels.
Why wrong: BFD over tunnels is not supported regardless of global configuration.
D
The BFD timers must be increased for tunnel interfaces.
Why wrong: Timers do not enable BFD over unsupported interfaces.

Full breakdown with real-world context →

Question 10mediummultiple choice

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A network engineer runs the following command to troubleshoot a Route Summarization issue:

R1# show ip access-lists CoPP-ACL

extended IP access list CoPP-ACL

10 permit eigrp any any (100 matches)
    
20 permit ospf any any (50 matches)
    
30 permit bgp any any (200 matches)
    
40 deny ip any any (0 matches)

What does this output indicate?

A
CoPP is permitting routing protocol traffic, including EIGRP, OSPF, and BGP, which are used for route summarization. No drops indicate CoPP is not blocking summarization.
The permit statements with matches show that routing protocol traffic is allowed, and no drops mean CoPP is not interfering.
B
CoPP is dropping all routing protocol traffic, preventing route summarization.
Why wrong: The deny statement has 0 matches, so no traffic is being dropped.
C
CoPP is only allowing BGP traffic, blocking EIGRP and OSPF summarization.
Why wrong: All three protocols have matches, indicating they are permitted.
D
CoPP is not configured because the access list is empty.
Why wrong: The access list has entries with matches.

Full breakdown with real-world context →

Question 11mediummultiple choice

Review the full OSPF breakdown →

A network engineer runs the following command on Router R1:

R1# show ip route 192.168.1.0

Routing entry for 192.168.1.0/24 Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 100 Redistributing via ospf 1 Last update from 10.1.1.2 on GigabitEthernet0/1, 00:00:05 ago Routing Descriptor Blocks:

* 10.1.1.2, from 10.1.1.2, 00:00:05 ago, via GigabitEthernet0/1

Route metric is 20, traffic share count is 1

Based on this output, what is the problem?

A
The route is an external OSPF route with metric 20, which is the default for Type 2 external routes.
Why wrong: This is correct, but not a problem.
B
The route is being redistributed back into OSPF, which could cause a routing loop if not filtered.
Redistributing a route learned from OSPF back into OSPF without filtering can cause loops.
C
The forward metric is 100, which is the metric to the ASBR.
Why wrong: The forward metric is normal for external routes.
D
The route is learned from neighbor 10.1.1.2, which is the next hop.
Why wrong: This is normal.

Full breakdown with real-world context →

Question 12hardmultiple choice

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OSPF network type mismatch on a multi-access link is causing route summarization issues. Router R1 and R2 are connected via Ethernet, but R1 has:

interface GigabitEthernet0/0
 ip address 10.0.0.1 255.255.255.0
 ip ospf network point-to-point
 ip ospf 1 area 0

!

Router R2 has default OSPF network type (broadcast). R1 is configured with:
router ospf 1
 area 0 range 10.0.0.0 255.255.255.0

! R2 shows:

R2# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.1          0   FULL/  -        00:00:30    10.0.0.1        GigabitEthernet0/0

But R2 does not have the summary route in its routing table. What is the root cause?

A
R1 is not an ABR (only area 0), so the area range command does not generate a summary route.
Area range only works on ABRs that connect multiple areas.
B
The network type mismatch causes OSPF to not exchange LSAs correctly, preventing the summary.
Why wrong: The adjacency is FULL, so LSAs are exchanged.
C
The summary route is suppressed because the interface is point-to-point.
Why wrong: Point-to-point does not suppress summarization.
D
R2 has a static route that overrides the summary.
Why wrong: No static route is indicated.

Full breakdown with real-world context →

Question 13mediummultiple choice

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A network engineer is troubleshooting an OSPFv2 route flapping issue. The router R1 is learning a route to 192.168.1.0/24 via two different paths: one through R2 and one through R3. The route is flapping between the two paths every few seconds. 'show ip ospf interface' shows that both interfaces are stable. What is the most likely cause?

A
The routers R2 and R3 have the same OSPF router ID.
Duplicate router IDs can cause OSPF to see the same route from two different neighbors as different, leading to route flapping.
B
The route is being redistributed by both R2 and R3 with different metrics.
Why wrong: Different metrics would cause the router to prefer one path, not flap.
C
The OSPF network type on the interfaces is set to broadcast, causing DR/BDR instability.
Why wrong: If the interfaces are stable, the DR/BDR election is stable.
D
The link between R1 and R2 has a high error rate causing intermittent packet loss.
Why wrong: The stem says the interfaces are stable, indicating no Layer 1 issues.

Full breakdown with real-world context →

Question 14hardmulti select

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An engineer must enable BFD for an OSPF single-hop session between two directly connected routers. Which TWO configuration changes are required on each router? (Choose TWO.)

A
Configure the bfd interval command under the interface.
Correct. The bfd interval command enables BFD on the interface and sets the timers.
B
Configure the ip ospf bfd command under the interface.
Correct. This command enables BFD support for OSPF on that specific interface.
C
Configure the bfd neighbor command under global configuration.
Why wrong: Incorrect. The bfd neighbor command is used for static BFD sessions, not for dynamic protocols like OSPF.
D
Configure the router ospf command and then use the bfd all-interfaces command.
Why wrong: Incorrect. While bfd all-interfaces enables BFD on all OSPF interfaces, it still requires the interface-level bfd interval command to be present; this option alone is insufficient.
E
Configure the bfd slow-timers command under the interface.
Why wrong: Incorrect. The bfd slow-timers command is used for multihop BFD sessions and is not required for single-hop OSPF BFD.

Full breakdown with real-world context →

Question 15mediummultiple choice

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A network engineer runs the following command to troubleshoot an OSPF adjacency issue:

R1# debug ip ospf adj
*Mar  1 00:12:34.567: OSPF-1 ADJ   RtrA: Interface GigabitEthernet0/0 going Up
*Mar  1 00:12:34.568: OSPF-1 ADJ   RtrA: 2 Way Communication to 10.1.1.2 on GigabitEthernet0/0, state 2WAY
*Mar  1 00:12:34.570: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Our router ID 1.1.1.1, his router ID 2.2.2.2
*Mar  1 00:12:34.571: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Neighbor is not DR, state 2WAY
*Mar  1 00:12:34.572: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: DR is 10.1.1.2, BDR is 10.1.1.1
*Mar  1 00:12:34.573: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Build the Start DBD
*Mar  1 00:12:34.574: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Master/Slave negotiation done
*Mar  1 00:12:34.576: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Exchange done, loading started
*Mar  1 00:12:34.578: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Loading done
*Mar  1 00:12:34.580: OSPF-1 ADJ   RtrA: NBR 10.1.1.2: Full

What does this output indicate?

A
The adjacency failed due to a mismatch in the DR election.
Why wrong: The debug shows successful DR/BDR election and adjacency reaching Full state.
B
The adjacency formed successfully, and the neighbor is in Full state.
The sequence of messages shows all OSPF adjacency steps completing, ending with 'Full'.
C
The adjacency is stuck in Exstart state due to MTU mismatch.
Why wrong: The debug shows Master/Slave negotiation done and Exchange done, so no MTU issue.
D
The neighbor is not the DR, so the adjacency will not form.
Why wrong: OSPF adjacencies form regardless of DR/BDR status; the debug shows Full state.

Full breakdown with real-world context →

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