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
