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OSPF Troubleshooting Scenarios

Practise 350-401 ENCOR 350-401 OSPF questions covering neighbour states, router IDs, areas, timers, passive interfaces, OSPF cost, route selection, and command-output troubleshooting.

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15
scenario questions
350-401
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Cisco
vendor

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 1hardmultiple choice
Review the full OSPF breakdown →

Refer to the exhibit. R1 has two equal-cost OSPF E2 routes to 10.1.1.0/24 via two different next hops. However, when tracing to 10.1.1.1, all traffic uses the path through 10.0.1.2. What is the most likely reason?

Exhibit

Refer to the exhibit.
R1# show ip route | include 10.1.1.0
O E2 10.1.1.0/24 [110/20] via 10.0.1.2, 00:00:34, GigabitEthernet0/0
O E2 10.1.1.0/24 [110/20] via 10.0.2.2, 00:00:34, GigabitEthernet0/1
R1# show ip ospf interface GigabitEthernet0/0 | include Cost
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
R1# show ip ospf interface GigabitEthernet0/1 | include Cost
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 100
R1# traceroute 10.1.1.1 source Loopback0
Type escape sequence to abort.
Tracing the route to 10.1.1.1
  1 10.0.1.2 4 msec 4 msec 4 msec
  2 10.0.3.2 8 msec 8 msec 8 msec
Question 2hardmulti select
Review the full OSPF breakdown →

Which THREE conditions are required for OSPF routers to become fully adjacent? (Choose three.)

Question 3easymultiple choice
Review the full OSPF breakdown →

Refer to the exhibit. An administrator needs to ensure that traffic to 192.168.1.0/24 is forwarded via a different path than traffic to 192.168.2.0/24, even though both routes are learned via OSPF with the same metric. Which action should the administrator take?

Exhibit

Refer to the exhibit.

R1# show ip route
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 10.0.0.2 to network 0.0.0.0

O E2     192.168.1.0/24 [110/20] via 10.0.0.2, 00:00:34, GigabitEthernet0/0
O E2     192.168.2.0/24 [110/20] via 10.0.0.2, 00:00:34, GigabitEthernet0/0
S*       0.0.0.0/0 [1/0] via 10.0.0.1
Question 4hardmultiple choice
Review the full OSPF breakdown →

A network administrator is troubleshooting an issue where OSPF routes are not being learned from a neighbor. The administrator checks the OSPF configuration and sees that both routers are in the same area. The neighbor state is stuck in EXSTART. What is the most likely cause?

Question 5mediummultiple choice
Review the full OSPF breakdown →

Examine the following configuration snippet:

interface GigabitEthernet0/1
 ip address 192.168.1.1 255.255.255.0
 ip ospf hello-interval 20
 ip ospf dead-interval 80

What is the effect of this configuration?

Question 6mediummultiple choice
Review the full OSPF breakdown →
Router R3 has the following OSPF configuration:

router ospf 1

router-id 3.3.3.3

network 10.0.0.0 0.255.255.255 area 0

default-information originate always metric 20 metric-type 2

What is the effect of the 'default-information originate always' command?

Question 7mediumdrag order
Review the full OSPF breakdown →

Drag and drop the steps of OSPF virtual link configuration into the correct order, from first to last.

Question 8mediumdrag order
Review the full OSPF breakdown →

Drag and drop the steps of OSPF neighbor adjacency formation into the correct order, from first to last.

Question 9harddrag order
Review the full OSPF breakdown →

Drag and drop the steps of OSPF route redistribution into a different autonomous system into the correct order, from first to last.

Question 10easymultiple choice
Review the full OSPF breakdown →

A network engineer is troubleshooting an automated configuration change that caused a routing loop. The change was pushed via an Ansible playbook that modified OSPF cost values on multiple routers simultaneously. What is the most likely reason for the loop?

Question 11hardmultiple choice
Review the full OSPF breakdown →

A large enterprise has a campus network with a collapsed core design. The core switch connects to two distribution switches, each serving several access switches. The network uses OSPF as the IGP. Recently, after a link failure between the core and distribution switch A, the network experienced a 30-second outage before converging. The engineer wants to improve convergence time to under 5 seconds. The budget is limited, so hardware upgrades are not an option. The engineer is considering the following actions: A. Enable OSPF Fast Hello on all interfaces. B. Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds. C. Implement OSPF LSA throttling with a minimum interval of 0 ms. D. Use OSPF incremental SPF (iSPF).

Which action will provide the most significant improvement in convergence time for this scenario?

Question 12hardmultiple choice
Review the full OSPF breakdown →

An enterprise is using OSPF in a multi-area design. Area 1 is a regular area, and Area 2 is a totally stubby area. Which LSA types are present in Area 2?

Question 13mediummultiple choice
Review the full OSPF breakdown →

An engineer is designing an MPLS L3VPN service for a customer that requires overlapping IP addresses between two sites. The customer uses OSPF as the PE-CE protocol. The engineer configures VRFs on the PE routers and assigns unique route distinguishers (RDs) and route targets (RTs). However, the customer reports that routes from one site are not being installed in the other site's VRF. What is the most likely cause?

Question 14hardmultiple choice
Review the full OSPF breakdown →

An engineer is troubleshooting a network where OSPF neighbors are stuck in the EXSTART state. What is the most likely cause?

Question 15mediummultiple choice
Review the full OSPF breakdown →

An engineer is troubleshooting an MPLS VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are running OSPF with the CE routers. On PE1, the 'show ip route vrf CUSTOMER' output shows 10.2.2.0/24 as an OSPF route, but the prefix is not present in the global BGP table. What is the most likely cause?

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