LSA Type 3 — Summary LSA - The Very Selective Gossip

So far we have met LSA 1, who only wanted to talk about himself, and LSA 2, who was The Chosen One.

Now it is time for LSA 3: The Gossip. Or even better, The Selective Gossip. She knows things. Probably a lot of things, but she only shares what she thinks you need to know.

LSA 3 knows about networks in another area, but don't expect all the details. She will only tell you what networks are over there and how far away they are. What is actually going on over there, you might wonder? Well, that's not really her concern.

Type 3 is called a Summary LSA and it is created by an Area Border Router, or ABR. The ABR uses Type 3 LSAs to tell routers in one area about networks that exist in another area.

You get enough information to know where to go, but you don't get the whole story.

And this is where LSA 3 is different from our first two characters. LSA 1 and LSA 2 describe what is happening inside their own area. An ABR uses LSA 3 to tell another area about networks that exist elsewhere in the OSPF domain

So if you only remember three things about LSA 3:

1. It is generated by an ABR.
2. It tells one area about networks in another area.
3. It gives you the destination and the cost, but not all the details from the other area.

The Selective Gossip spreads the news to other areas. She just refuses to share all the gossip.

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

An ABR can advertise a network learned in Area 10 into Area 0 as a Type 3 LSA.