LSA Type 5 — External LSA - The Dossier

So, LSA 4 told us: “I know a guy.” Well, today we finally meet that guy.

Remember Oskar and Mikael? Oskar wanted to reach Mikael, who lives outside of OSPF. LSA 4 helped Oskar figure out how to reach the ASBR, the router that knows about routes outside the OSPF domain.

But knowing where the ASBR is doesn't actually tell Oskar where Mikael lives. For that, we need LSA 5.

LSA 5 is The Dossier.

A Type 5 LSA, also called an External LSA, is created by an ASBR when external routes are redistributed into OSPF. It carries information about those external networks so other LSA 5 capable areas in the OSPF domain can learn about them.

Basically, the ASBR shows up with a dossier and says:

I know how to get to Mikael. And here is the information you need.

That dossier is the LSA 5.

This also explains why LSA 4 and LSA 5 work so well together. LSA 4 tells you how to reach the guy who knows about the external route. LSA 5 contains the information about the external route itself.

So if you only remember three things about LSA 5:

1. It is generated by an ASBR.
2. It advertises routes that came from outside OSPF.
3. It carries the external route information, while LSA 4 helps other areas reach the ASBR advertising it.

Yesterday LSA 4 said: I know a guy. Today that guy sends you the Dossier.

Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.

A controlled redistribution policy can introduce a static prefix as external OSPF information. Never redistribute everything merely to make one route appear.