CCNA OSPF interview questions and answers should test practical understanding of link-state routing, neighbor discovery, databases, SPF calculation, timers, and route verification. The answers below correct several common OSPF misconceptions.
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Which algorithm does OSPF use?
OSPF uses Dijkstra’s Shortest Path First algorithm. Each router builds a link-state database and runs SPF to calculate the lowest-cost paths.
What are the main OSPF data structures?
OSPF maintains a neighbor table, a link-state database, and routes selected for installation in the routing table. The neighbor table tracks adjacencies, while the database represents the area topology.
How does OSPF discover neighbors?
OSPF routers send Hello packets. Neighbors must agree on important settings such as area ID, Hello and Dead intervals, authentication, network type, and compatible addressing.
What are the default Hello and Dead timers?
On common broadcast and point-to-point networks, the default Hello interval is 10 seconds and the Dead interval is 40 seconds. Other network types may use different defaults.
Does an OSPF Hello packet have a fixed size?
No single fixed byte size should be memorized for every Hello packet. Its size depends on the fields, options, authentication, and number of listed neighbors.
When does OSPF exchange routing information?
After neighbors form an adjacency, they synchronize link-state databases. OSPF then sends event-driven link-state updates when topology information changes, rather than periodically sending the entire routing table.
What are partial and incremental updates?
These terms describe sending only relevant changed link-state information instead of retransmitting all known routes. OSPF still refreshes LSAs periodically to maintain database validity.
How often are OSPF LSAs refreshed?
Self-originated LSAs are normally refreshed every 30 minutes, but this is not the same as exchanging the complete topology table every 30 minutes.
What is OSPF cost?
Cost is the metric OSPF uses to compare paths. Interface cost is based on reference bandwidth divided by interface bandwidth, unless the cost is configured manually.
What is the role of Area 0?
Area 0 is the OSPF backbone. Other areas normally connect to it through area border routers so inter-area traffic can be exchanged consistently.
What are DR and BDR?
On broadcast multi-access networks, the Designated Router and Backup Designated Router reduce the number of full adjacencies and organize LSA exchange.
What is an OSPF router ID?
The router ID is a 32-bit identifier for the OSPF process. A manually configured router ID provides predictable operation and simplifies troubleshooting.
Which commands verify OSPF?
Use show ip ospf neighbor, show ip ospf interface, show ip ospf database, show ip route ospf, and show ip protocols.
Why might an OSPF adjacency fail?
Typical causes include area mismatch, timer mismatch, authentication mismatch, subnet problems, duplicate router IDs, passive interfaces, MTU mismatch, and ACL filtering.
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