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OSPF Interview questions - Part 1


What is the protocol number of OSPF?





89





What is the multicast address of OSPF messages?





  • 224.0.0.5 for OSPF All-routers multicast
  • 224.0.0.6 for OSPF All-Designated routers multicast




What are the OSPF hello packet timers?





Hello packets are sent every 10 seconds with the Dead timer being 40 seconds





Is it mandatory to have OSPF process ID the same on adjacent routers to form OSPF neighborship?





No





What is the impact of interface MTU mismatch on adjacent OSPF routers?





MTU is not checked during the formation of neighbor adjacency. However, mismatched MTU values impede in the successful exchange of Database Description (DD) packets and thereby prevent the neighbors from reaching the "FULL" neighborship state.





What are the types of OSPF LSAs?





  • Type 1 - Router LSA
  • Type 2 - Network LSA
  • Type 3 - Summary LSA
  • Type 4 - ASBR Summary LSA
  • Type 5 - External LSA
  • Type 6 - Multicast OSPF (MOSPF) LSA
  • Type 7 - Not-So-Stubby-Area (NSSA)LSA
  • Type 8 - External Attribute LSA for BGP




Who generates Type-1 LSA?





Type 1 LSAs (router link states) are created by each router in the area to describe itself, its interfaces and neighbors.





What OSPF network types does DR/BDR election occur?





DR and BDR election occurs only on NBMA and Broadcast network types





Where are Type 2 LSAs created?





Type 2 - Network LSAs are only created for networks where a DR has been elected.





Who generates Type 4 LSA?





Type 4 LSA's are generated by ABRs which have an ASBR in the area it touches with. Type 4 LSA is created by ABR to describe the ASBR and its cost to reach it.





What does a Type 3 LSA contain?





Type 3 LSA's contain the minimum information including:





  • i. The subnet
  • ii. The mask
  • iii. The cost which is known by the ABR to reach it




What mechanism does OSPF provide to reduce excessive traffic due to LSA re-flooding in large networks?





The LSA aging process can be disabled by setting the LSA - "Do Not Age" bit. This is an interface configuration as below. Post enabling this command, the periodic LSA re-flooding won't be necessary.






interface fa0/0
ip ospf flood-reduction






How can you prevent DR election on broadcast links (Ethernet)? / How can you prevent Type 2 LSAs being generated on broadcast links?





Configure the link as "point-to-point" OSPF network type.. This prevents DR election and Type 2 LSA generation.


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