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OSPF Interview Questions - Part 2



What are types of OSPF packets?





  • Hello
  • Database Description (DBD)
  • Link-State Requests (LSR)
  • Link-State Updates (LSU)
  • Link-State Acknowledgements (LSAck)






What is OSPF finite state machine? What are the states in OSPF neighborship?





  • DOWN - No OSPF packets have been received on the interface
  • ATTEMPT - Applies only to non-broadcast multi-access networks (NBMA) where the neighbors must be configured manually using the "neighbor x.x.x.x" command
  • INIT - Router has seen a Hello message from an OSPF router
  • TWO-WAY - A Hello message has been sent to the neighbor and the neighbor has replied with its Hello message. In Broadcast networks, the DR/BDR election takes place after this state
  • EXSTART - DR and BDR establish OSPF adjacencies with each of the routers in the network. Master / Slave election takes place at this stage. The Master sends its DBD (Database Descriptor) first
  • EXCHANGE - Routing information may be exchanged via DBD, Link State Requests (LSR) and Link State Updates (LSU)
  • LOADING - LSRs are sent to each of the neighbors for the networks that the router is not aware of. LSUs are sent in response to provide information about the requested networks
  • FULL - All the link state databases are synchronized and the OSPF adjacencies have been successfully established.





Which parameters must match for the OSPF adjacency to come up?





  • Subnet
  • Area ID
  • Hello and Dead interval
  • Authentication




What are OSPF area types?





  • Backbone area
  • Non-backbone area
  • Stub
  • Totally stubby NSSA (not so stubby area)
  • Totally NSSA (not so stubby area)




What is the OSPF router-ID selection criteria?





  • Manually configuring the Router ID
  • Highest IP address of the loopback interfaces
  • Highest IP address of the non-loopback interfaces




What is ABR in OSPF?





Area Border Router is a router that connects other areas (non-backbone) to the backbone area. Unlike IS-IS L1-L2 router, OSPF ABRs have their interfaces in more than one area. The ABR, in fact serves as a demarcation between multiple areas.





What is ASBR in OSPF?





An Autonomous System Border Router (ASBR) connects different autonomous systems with each other. In case of OSPF, it is the router where the redistribution of some other protocol into OSPF takes place.


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