I have been around OSPF for some time now, but I had not until recently (while studying for ENCOR), done a deep dive into understanding how to read and understand the link state database (LSDB). The LSDB essentially contains route reachability information for all known OSPF routes in a network and is built from six different link state advertisement (LSA) types, which are listed and described (with my notes) here.

  • Type 1 – Router LSA
    • Each router in an area generates a single Type 1 LSA that describes all OSPF enabled links.
    • The foundation LSA of OSPF.
    • Type 1 LSAs do not get advertised outside of the area of origin.
  • Type 2 – Network LSA
    • Type 2 LSAs describe the network and attached routers of multi-access links.
    • Like Type 1 LSAs, Type 2 LSAs are not advertised outside of the area or origin.
  • Type 3 – Summary LSA
    • Generated at area border routers (ABRs).
    • Describe networks that originate from another area.
    • Type 1/2 LSAs are recreated at ABRs as Type 3 LSAs to be advertised to other areas.
  • Type 4 – Summary ASBR LSA
    • Generated at area border routers (ABRs).
    • Describe how to reach an autonomous system boundary router (ASBR).
    • Used in conjunction with Type 5 LSAs.
      • Because Type 5 LSAs describe networks that are advertised by ASBRs, routers need to know how to reach the ASBR. The Type 4 LSA is advertised by the ABR to the local area so the local routers know that to reach an ASBR, they need to route to the ABR.
  • Type 5 – AS external LSA
    • Describe external routes that are redistributed into OSPF.
    • Generated at autonomous system boundary routers (ASBRs).
    • The only LSA type that is advertised “in tact” throughout multiple areas.
  • Type 7 – NSSA LSA
    • Describe routes that are redistributed by an autonomous system boundary router within a Not-So-Stubby Area.

The LSDB can be a very powerful tool for discovery. Because all routers within an area maintain and identical LSDB, you can draw out a topology map of that entire area by just logging into an looking at the LSDB of a single router. When doing this, you would just need to focus on the type 1 (router) and type 2 (network) LSAs.

  • Suggested commands
    • show ip ospf database
    • show ip ospf database router
    • show ip ospf database network

Published by Tim Bertino

Systems Architect passionate about solutions and design.

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