An OSPF Type 5 Link State Advertisement (LSA), also known as an External LSA, that contains information about routes that are external to the OSPF domain. Such an LSA is generated by ASBRs and propagated from area to area by ABRs.

Take a look at this diagram:


In the above diagram, you can see that R1 is an ASBR because it is connected to a RIP domain, which resides outside of the OSPF routing domain. Information from that RIP domain is learned by R1 and is injected into OSPF Area 2 using a Type 5 LSA.

This LSA is then propagated via ABRs from area to area. In this case, R2 injects it into Area 0 and R3 injects it into Area 51.

In large networks, Type 5 LSAs can create very large Link State Databases (LSDBs). For this reason, the idea of an OSPF Stub area has been introduced that allows and disallows various types of LSAs. In particular, stub areas will not allow External LSAs within that area.

For this reason, OSPF NSSAs have been introduced along with Type 7 LSAs to allow ASBRs to operate within a stub network.




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