LSA Type 4 — ASBR Summary LSA - I know a guy

So far we have met the Narcissist, the Chosen One and the Selective Gossip. Now it is time for LSA 4. And I think we can call him: I Know a Guy. Let me explain why:

Imagine that Oskar wants to reach Mikael, who lives outside of OSPF. There is a router that knows how to get to Mikael. That router is an Autonomous System Boundary Router, or ASBR. An ASBR is basically the router that connects OSPF to routes coming from somewhere outside the OSPF domain.

But there is a problem. What if Oskar is in another area and doesn't know how to reach the ASBR?

This is where LSA 4 comes in. A Type 4 LSA, also called an ASBR Summary LSA, is created by an ABR. Its job is to tell routers in other areas how to reach the ASBR.

It does NOT tell you about the external networks themselves, that is someone else's job, and we will get to that very soon.

LSA 4 is basically saying:

I don’t know how to get to Mikael. But I know a guy who does.

So if you only remember three things about LSA 4:

1. It is generated by an ABR.
2. It tells routers how to reach an ASBR in another area.
3. It does not describe the external routes themselves. It only points you toward the router that knows about them.

LSA 4 doesn't know everything. He just knows a guy wow knows stuff.

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

A router receiving external route information may also need to know how to reach the advertising ASBR. Type 4 supplies the inter-area ASBR reachability part of that story.