Top 25 OSPF Interview Questions and Answers

1. What is OSPF Routing Protocol?
Open shortest path first is an Open Standard Link State routing protocol which works by using Dijkastra algorithm to initially construct the shortest paths and follows that by populating the routing table with resulting best paths.

2. What are the steps required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol).
2.Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets.
After Database synchronization is complete, the two routers are considered adjacent.

3. Explain LSA (Link-State Advertisement), LSU (Link State Update) and LSR (Link State Request)?
The LSAs (Link-State Advertisements) are used by OSPF routers to exchange routing and topology information. When two neighbors decide to exchange routes, they send each other a list of all LSAs in their respective topology database. Each router then checks its topology database and sends Link State Request (LSR) message requesting all LSAs that was not found in its topology table. Other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.

4. Explain OSPF Router ID?
Router Id is used to identify the Router. Highest IP address of the router's loopback interfaces is chosen as the Router ID, If no loopback is present than highest IP address of the router's physical interfaces will be chosen as Router ID. OSPF prevents neighborships between routers with duplicate RIDs. All OSPF RIDs in a domain should be unique. OSPF Router ID should not be changed after the OSPF process is started and the OSPF neighborships are established. If you change the OSPF router ID, we need to either reload the IOS or use "clear ip ospf process" command (restart the OSPF process) for changed RID to take effect.
To manually configure the router ID
R1(config)# router ospf 5
R1(config-router)# router-id

5. Can we use OSPF without backbone area?
Yes, but than only intra-area communication is possible. Inter-area communication is not possible without backbone area.

Read More Questions:
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OSPF Interview Questions Part5