OSPF Interview Questions and Answers
Intermediate / 1 to 5 years experienced level questions & answers
Ques 1. Explain the OSPF area types.
OSPF supports different area types, including backbone area (Area 0), regular areas, stub areas, totally stubby areas, and not-so-stubby areas (NSSA).
Ques 2. How does OSPF prevent routing loops?
OSPF uses the Dijkstra algorithm to calculate the shortest path tree, and it avoids routing loops by using the concept of a designated router (DR) on multi-access networks.
Ques 3. Explain the OSPF LSDB (Link State Database).
The LSDB contains information about the network's topology, represented as link-state advertisements (LSAs). Routers use this database to calculate the SPF tree.
Ques 4. Differentiate between OSPF areas and autonomous systems.
OSPF areas are subdivisions of an OSPF autonomous system. An OSPF autonomous system is a collection of OSPF areas connected by a backbone area (Area 0).
Ques 5. How does OSPF handle equal-cost paths?
OSPF allows for load balancing across equal-cost paths by installing multiple routes in the routing table.
Ques 6. What is OSPF metric, and how is it calculated?
OSPF metric is cost, which is inversely proportional to bandwidth. The formula is cost = 10^8 / bandwidth (in bps).
Ques 7. Explain OSPF DR/BDR election process.
On multi-access networks, OSPF routers elect a Designated Router (DR) and a Backup Designated Router (BDR) to reduce OSPF routing traffic.
Ques 8. What is the OSPF LSAge timer?
The LSAge timer in OSPF is used to age out LSAs from the LSDB. If an LSA's age exceeds its maximum age, it is considered expired and removed from the LSDB.
Ques 9. What is OSPF NSSA (Not-So-Stubby Area)?
NSSA is an OSPF area type that allows injection of external routes while still maintaining some stub area characteristics.
Ques 10. Explain OSPF path selection criteria.
OSPF selects paths based on the OSPF metric (cost). Lower cost paths are preferred. If costs are equal, the router considers other tie-breaker criteria.
Ques 11. What is OSPF Type 3 LSA (Summary LSA)?
Type 3 LSAs are used to advertise summarized routes from one area to another. They are generated by the Area Border Router (ABR).
Ques 12. What is OSPF External LSA (Type 5 LSA)?
Type 5 LSAs represent external routes injected into OSPF. They are generated by Autonomous System Boundary Routers (ASBRs).
Ques 13. Explain OSPF route summarization.
Route summarization in OSPF involves aggregating multiple routes into a single summarized route, reducing the size of the routing table.
Ques 14. Explain OSPF router types.
OSPF routers can be designated as internal routers, area border routers (ABRs), autonomous system boundary routers (ASBRs), or backbone routers.
Ques 15. Explain OSPF stub area characteristics.
In OSPF stub areas, routers do not receive external routes, reducing the LSDB size. The area relies on a default route for external destinations.
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