The purpose of the Type-4 LSA in OSPF is not always readily perceivable.

The LSA type 4 is used when you have multiple areas as well as an Autonomous System Border Router (ASBR).

First of all, an ASBR is a router that has at least one interface in an OSPF area (area 0 or otherwise) and at least one interface in another Autonomous System (i.e. a region of your network running another routing protocol, static routing or another autonomous OSPF implementation).

The ASBR sends type 5 LSAs (external routes) to its OSPF area(s). The type 5 LSAs are flooded to all areas. However, they do not contain the necessary information to FIND the ASBR. The ABRs (which connect OSPF areas) which receive these type 5 LSAs will also send out a type 4 LSA to all other areas to inform all routers of the location of the ASBR.

So, the type 4 LSA** informs all other areas of the location** (or the IP address) of the ASBR so they know the path that must be taken to reach the external routes advertised by the type 5 LSAs.

For information about other types of LSAs, take a look at OSPF LSA Types.



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