OSPF Link State Advertisements (LSAs)

When OSPF routers become neighbours, they synchronise their Link State Databases between them. If a OSPF router adds or removes a directly connected route from its database, it floods a Link State Advertisment (LSA) out of the OSPF enabled interfaces. There are six types of LSAs that are used in an OSPF network:

TypeLSA NameDescription
1RouterThe LSAs that are within an area
2NetworkA multi-access network attached to a designated router
3SummaryA network prefix that had originated from a different area
4ASBRA summary LSA for a autonomous system boundry router (ASBR)
5AS externalA LSA for a route that had been redistributed into OSPF
7NSSARedistribute routes in not so stubby areas
The six types of link state advertisement

LSA Sequence Numbers

LSAs have sequence numbers in them to help prevent any problems where an LSA may become delayed in network transmission.

The sequence number is a 32-bit value that is incremented each time a LSA is sent out.

If a router receives an LSA greater than the one in the LSDB, it processes it. If the LS is less than that in the LSDB, it is discarded due to it being observed as outdated.

LSA Aging

Each OSPF LSA entered into the Link State Database has an age that increments by 1 every second.

Once the age reaches 30 minutes (1800 seconds) for the networks, the originating router advertises a new LSA with the age set back to 0. Each router will forward this LSA, and it is incremented with a calculated delay that reflects the link.

If the LSA age reaches 3600, the LSA is classified as invalid and is dropped from the link state database.

This ensures the OSPF LSDB has fresh routes, and not anything stale.



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