OSPF Interview Questions 2025 – 45 Most Asked Questions

Friends OSPF (Open Shortest Path First) is one of the most important routing protocols used in computer networks. It helps different routers communicate and find the best path for data to travel. If you are preparing for an interview in 2025 and need to answer OSPF-related questions, this guide will help you understand the key concepts in a simple way. We will cover basic to advanced questions to make sure you are well-prepared. Let’s get started!

Most Asked OSPF Interview Questions 2025

1. What is OSPF, and why is it used?

Answer: OSPF (Open Shortest Path First) is a link-state routing protocol used to find the best path for data packets in IP networks.
Explanation: It uses the Dijkstra algorithm to calculate the shortest path and is widely used in enterprise and service provider networks due to its scalability and fast convergence.

2. How does OSPF work?

Answer: OSPF creates a network topology by exchanging link-state information between routers and then calculates the best path using the shortest path first (SPF) algorithm.
Explanation: It builds a link-state database (LSDB) and updates it dynamically to ensure efficient routing.

3. What are the different OSPF packet types?

Answer: The five OSPF packet types are:

1. Hello – Used to discover and maintain neighbor relationships.
2. Database Description (DBD) – Contains summaries of the LSDB.
3. Link-State Request (LSR) – Requests specific LSAs from a neighbor.
4. Link-State Update (LSU) – Sends updated LSAs to neighbors.
5. Link-State Acknowledgment (LSAck) – Confirms the receipt of LSAs.

4. What is an OSPF area, and why is it important?

Answer: An OSPF area is a logical grouping of routers that reduce the size of the routing table and improve efficiency.
Explanation: By dividing the network into multiple areas, OSPF reduces overhead and speeds up convergence. The backbone area (Area 0) connects all other areas.

5. What is the difference between OSPFv2 and OSPFv3?

Answer: OSPFv2 is used for IPv4 networks, while OSPFv3 is designed for IPv6 networks.
Explanation: OSPFv3 supports IPv6 address prefixes, enhanced security, and better efficiency in handling multicast traffic.

6. What are the different OSPF router types?

Answer: The four types of OSPF routers are:

• Internal Router (IR): Exists within a single area.
• Backbone Router (BR): Located in Area 0.
• Area Border Router (ABR): Connects two or more areas.
• Autonomous System Boundary Router (ASBR): Connects OSPF with other routing protocols.

7. What is an OSPF neighbor, and how does OSPF form neighbor relationships?

Answer: OSPF neighbors are routers that establish communication to exchange routing information.
Explanation: Routers become neighbors by exchanging Hello packets, matching their parameters like area ID and subnet, and then progressing through OSPF states.

8. What are OSPF states, and what do they mean?

Answer: The OSPF states are:

• Down: No Hello packets received.
• Init: Hello packets received but not acknowledged.
• Two-Way: Both routers have acknowledged each other.
• ExStart: Master-slave relationship established.
• Exchange: Database Description (DBD) packets are exchanged.
• Loading: Link-State Requests (LSRs) and Link-State Updates (LSUs) are exchanged.
• Full: The LSDB is fully synchronized.

9. How does OSPF calculate the best path?

Answer: OSPF uses the Dijkstra algorithm (Shortest Path First – SPF) to determine the best route based on cost (metric).
Explanation: The cost is calculated as cost = reference bandwidth / interface bandwidth (default reference bandwidth is 100 Mbps).

10. What is an OSPF LSA (Link-State Advertisement)?

Answer: An LSA is a packet containing routing and topology information.
Explanation: LSAs help OSPF routers build their link-state databases. Different LSA types (1-7) define various network topologies and functions.

11. What is the difference between OSPF and other routing protocols like RIP or EIGRP?

Answer:

• OSPF: Link-state protocol, fast convergence, supports large networks.
• RIP: Distance-vector protocol, slow convergence, limited to 15 hops.
• EIGRP: Hybrid protocol, faster than RIP but Cisco-proprietary.

12. What is OSPF Stub, Totally Stub, and NSSA (Not-So-Stubby Area)?

Answer:

• Stub Area: Blocks external routes but allows intra-area routes.
• Totally Stubby Area: Blocks both external and inter-area routes, only allowing a default route.
• NSSA: Allows limited external routes while still being a stub area.

13. What is the role of the OSPF Designated Router (DR) and Backup Designated Router (BDR)?

Answer: The DR is responsible for exchanging routing information with other routers, and the BDR takes over if the DR fails.
Explanation: This reduces network overhead on multi-access networks like Ethernet.

14. How can you troubleshoot OSPF network issues?

Answer:

1. Check neighbor relationships using show ip ospf neighbor.
2. Verify OSPF process settings with show ip ospf.
3. Check interface states using show ip ospf interface.
4. Check LSDB entries with show ip ospf database.
5. Use debug commands carefully (debug ip ospf events).

15. What are some best practices for optimizing OSPF performance?

Answer:

• Use proper area design (e.g., backbone and stub areas).
• Adjust OSPF timers to improve convergence.
• Summarize routes at ABRs and ASBRs.
• Avoid excessive LSA flooding with filtering and proper area design.
• Enable authentication to improve security.

Ospf Interview Questions For Experienced

Friends If you’re an experienced in network engineer preparing for an OSPF-related interview, these advanced-level questions will help you brush up on your knowledge.

16. What is the OSPF cost metric, and how is it calculated?

Answer: OSPF cost is calculated as Cost = Reference Bandwidth / Interface Bandwidth.
Explanation: By default, the reference bandwidth is 100 Mbps, so a 1 Gbps link has a cost of 1 (100/1000). The cost determines the best path selection.

17. How does OSPF handle load balancing?

Answer: OSPF supports equal-cost multi-path (ECMP), allowing multiple paths with the same cost to be used for load balancing.
Explanation: By default, Cisco routers allow up to 4 equal-cost paths, but this can be increased to 16 using maximum-paths.

18. What is the role of LSA types in OSPF?

Answer: OSPF has multiple LSA types, each serving a different function:

• Type 1 (Router LSA): Describes router links within an area.
• Type 2 (Network LSA): Describes multi-access networks (sent by DR).
• Type 3 (Summary LSA): Advertises inter-area routes (sent by ABR).
• Type 4 (ASBR Summary LSA): Provides a route to ASBR.
• Type 5 (External LSA): Advertises external routes from other protocols.
• Type 7 (NSSA LSA): Used in NSSA to carry external routes.

19. How do you change OSPF election results for DR and BDR?

Answer: Use the OSPF priority command:

interface GigabitEthernet0/1  
ip ospf priority 100  

Explanation: A higher priority increases the chance of becoming a DR or BDR. A priority of 0 prevents the router from becoming a DR/BDR.

20. What is OSPF LSA flooding, and how can you control it?

Answer: LSA flooding occurs when routers continuously exchange LSA updates, causing excessive overhead.
Solution:

• Use stub areas to limit external LSAs.
• Apply route summarization at ABRs.
• Use LSA filtering with the distribute-list command.

21. What is the difference between an ABR and an ASBR in OSPF?

Answer:

• ABR (Area Border Router): Connects multiple OSPF areas and distributes inter-area routes.
• ASBR (Autonomous System Boundary Router): Connects OSPF to external routing domains (e.g., BGP, EIGRP).

22. How does OSPF authentication work, and why is it important?

Answer: OSPF supports:

• Plaintext authentication (insecure)
• MD5 authentication (secure)
• SHA authentication (more secure)
  Explanation: Authentication prevents unauthorized routers from forming OSPF neighbor relationships and injecting false routes.

23. What is the OSPF database, and how can you verify it?

Answer: The OSPF database stores LSAs and is checked using:

show ip ospf database  

Explanation: This command helps verify LSAs, troubleshoot missing routes, and detect stale entries.

24. How does OSPF handle route summarization?

Answer:

• Inter-area summarization is done on ABRs using: area 1 range 192.168.0.0 255.255.252.0
• External route summarization is done on ASBRs using: summary-address 10.10.0.0 255.255.0.0

Explanation: Summarization reduces routing table size and speeds up convergence.

25. What is OSPF route filtering, and how do you implement it?

Answer: OSPF route filtering prevents unwanted routes from being advertised.

• Use a distribute-list to filter routes: distribute-list 10 in
• Use a route-map for fine-grained control.

26. What is the OSPF default route, and how is it advertised?

Answer:

• From ABR to other areas: area 1 default-cost 10
• From ASBR to inject an external default route: default-information originate

Explanation: This command ensures OSPF routers use a specific default route when no other matches exist.

27. What is the difference between stub, totally stubby, and NSSA areas?

Answer:

• Stub: Blocks external LSAs (Type 5).
• Totally Stubby: Blocks external LSAs and Type 3 (inter-area).
• NSSA (Not-So-Stubby Area): Blocks Type 5 but allows Type 7 (used for external routes).
  Explanation: These area types reduce overhead by controlling unnecessary LSA propagation.

28. How does OSPF handle network topology changes?

Answer:

• When a link fails: OSPF recalculates routes using the SPF algorithm and updates the LSDB.
• When a new link is added: OSPF sends Hello packets, updates LSAs, and recalculates paths.
  Explanation: OSPF converges quickly, but frequent topology changes can lead to high CPU usage.

29. How do you troubleshoot OSPF adjacency issues?

Answer: Check the following:

• Verify interface status: show ip ospf interface
• Check neighbor relationships: show ip ospf neighbor
• Look for mismatched parameters (area ID, MTU, authentication)
• Debug OSPF packets carefully: debug ip ospf adj

30. How does OSPF Graceful Restart (NSF) work?

Answer: OSPF Non-Stop Forwarding (NSF) prevents traffic loss during router reboots.

• Graceful Restart (RFC 3623) allows a router to restart without causing network-wide OSPF recalculations.
  Explanation: Neighbor routers temporarily retain the LSDB and wait for the restarting router to rejoin, preventing unnecessary SPF recalculations.

OSPF Scenario Based Interview Questions and Answers

31. Scenario: Two routers are connected, but OSPF neighbors are not forming. What could be the issue?

Answer:
Possible causes:

• Mismatched OSPF parameters (area ID, subnet mask, authentication, MTU).
• OSPF not enabled on the correct interfaces (network command missing).
• Physical layer issues (cable, interface down).
• ACLs or firewall blocking OSPF packets.
• Different OSPF network types (e.g., broadcast vs. point-to-point).

Solution:
Check configurations with:

pgsqlCopyEditshow ip ospf neighbor
show ip ospf interface
debug ip ospf adj

32. Scenario: Your OSPF network is converging slowly. How would you fix it?

Answer:
Slow convergence can be caused by:

• Large topology database → Use route summarization.
• High LSA flooding → Use stub/totally stubby areas.
• Frequent topology changes → Adjust OSPF timers.
• Slow SPF calculations → Check CPU utilization and optimize network design.

33. Scenario: OSPF routes are missing in the routing table. What steps would you take?

Answer:

1. Verify OSPF process (show ip ospf) to confirm it’s running.
2. Check neighbor relationships (show ip ospf neighbor).
3. Look at LSDB (show ip ospf database) to confirm LSAs exist.
4. Check route filtering (ACLs, distribute-list, route-maps).
5. Check administrative distance (if another routing protocol is preferred).

34. Scenario: An OSPF router is stuck in the “EXSTART” state. What could be the cause?

Answer:

• MTU mismatch between neighbors.
• Authentication mismatch.
• Master/slave negotiation failure due to packet loss.

Solution: Adjust MTU with:

kotlinCopyEditinterface GigabitEthernet0/1
ip mtu 1500

35. Scenario: Your OSPF network has high CPU usage. How do you troubleshoot?

Answer:

1. Check SPF recalculations (show ip ospf statistics).
2. Look for excessive LSA flooding (show ip ospf database).
3. Enable OSPF throttling to limit LSA frequency.
4. Summarize routes to reduce LSDB size.

36. Scenario: Some OSPF routers are receiving external routes, but others are not. Why?

Answer:

• Stub or Totally Stubby areas might be blocking Type 5 LSAs.
• OSPF filtering rules (distribute-list, route-map).
• ASBR not redistributing routes properly (redistribute command missing).

Solution: Check area types and route redistribution settings.

37. Scenario: An OSPF router is elected as DR when it shouldn’t be. How do you fix it?

Answer:

• DR/BDR election is based on highest priority, then highest RID.
• Set OSPF priority to 0 to prevent DR election:kotlinCopyEditinterface GigabitEthernet0/1 ip ospf priority 0
• Restart OSPF process for re-election.

38. Scenario: You need to advertise a default route into OSPF. How would you do it?

Answer:
From an ASBR:

cppCopyEditrouter ospf 1
default-information originate

Explanation: Injects a default route (0.0.0.0/0) into OSPF, useful for Internet access.

39. Scenario: OSPF adjacency is stuck in “2-Way” state. What does it mean?

Answer:

• Normal for non-DR/BDR routers in a broadcast network.
• If it should fully form, check:
  • Hello/dead timers mismatch.
  • Multicast filtering blocking Hellos (224.0.0.5).
  • Incorrect network type (e.g., point-to-multipoint instead of broadcast).

40. Scenario: Two OSPF areas are not exchanging routes. What could be wrong?

Answer:

• The ABR is missing area configurations (area range or network command).
• Area types are mismatched (e.g., one is stub, the other is normal).
• LSA filtering is blocking inter-area advertisements.

41. Scenario: OSPF is redistributing BGP routes, but some routers are not receiving them. Why?

Answer:

• BGP routes may have a high OSPF metric (use metric-type 1 or 2).
• Stub/totally stubby areas block external Type 5 LSAs.
• Redistribution is missing a default metric (use redistribute bgp 100 subnets metric 10).

42. Scenario: OSPF failover is slow. How can you improve it?

Answer:

• Enable OSPF Fast Hello (ip ospf dead-interval 1).
• Use BFD (Bidirectional Forwarding Detection) for faster link failure detection.
• Reduce LSA throttle timers.

43. Scenario: You need to summarize OSPF routes at an ABR. How do you do it?

Answer:
Use the area range command:

nginxCopyEditrouter ospf 1
area 1 range 10.10.0.0 255.255.252.0

Explanation: Summarization reduces routing table size and improves stability.

44. Scenario: OSPF routes are flapping frequently. How do you troubleshoot?

Answer:

• Check physical links (show interfaces for errors).
• Enable LSA pacing (timers throttle lsa all).
• Reduce SPF recalculations (timers throttle spf 10 100 500).
• Use OSPF stub areas to limit unnecessary LSA propagation.

45. Scenario: An OSPF router is advertising an incorrect subnet mask. How do you fix it?

Answer:

• Check interface subnet mask (show ip interface brief).
• Verify OSPF network statement in router ospf.
• If using redistribution, set the correct mask:arduinoCopyEditredistribute connected subnets
• Manually correct the subnet in OSPF configuration.

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