Router LSA Quiz: The Results

The Friday’s OSPF quiz has generated numerous answers … unfortunately many of them incorrect. Some readers (probably those that recently attended a Cisco certification exam) thought I was asking a trick question, as I’ve forgotten to include the IP addresses in the sample configuration, which only proves how hard it is to write good bulletproof questions.

Those that assumed the IP addresses would have to be configured on the interfaces made two common errors:

  • Some assumed a type-2 LSA would be generated for the LAN interface. Wrong: type-2 LSA is generated only if needed (there is more than one router attached to the LAN interface).
  • Others thought the router would generate a type-1 LSA per interface. Wrong: an OSPF router generates only a single type-1 LSA per area.

To clarify these issues, I wrote an article documenting how the type-1 (router) LSA describes various interface types and inter-router links.


  1. Ivan,

    Thanks for such a wonder document of LSA.

    shivlu jain
  2. It seems a router will originate a single router-LSA (considering the router is a part of single area) containing a single router-link (stub network; considering only one interface is enabled with ospf) whether or not adjacency is formed.

    However, what is the reason to advertise a stub link when an adjacency is formed? what information is provided to the neighbor router with this stub link? *DONT_KNOW*
  3. Does anything prevent Full adjacency between a broadcast interface on one end and point-to-point configured on the other? If I understand right Standard doesn't say anything on this or even packets don't carry any related information to prevent it.
  4. You need to adjust OSPF timers manually. I'm also not sure how the DR/BDR election would work in this case.
  5. 7 years late, I know, but...

    Ivan states above "type-2 LSA is generated only if needed (there is more than one router attached to the LAN interface)"

    To clarify, the type-2 Network LSA will only be originated by the DR of the LAN segment.
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