type 7 lsa and type 5 lsa confusion

hitmen

after another routing protovcol reaches the asbr, the asbr converts it into type 7 lsa. then the type 7 lsa is converted into type 5 lsa when it reaches the abr. am i right?

but the ocg textbook states that type 5 lsa are not accepted for nssa and stubby area. this is making me so confused

mistabrumley89

You are correct. It all depends on what kind of stub area is configured.
Maybe this chart will help ease your confusion.

mistabrumley89

What the OCG is saying is that those type 5 LSA's can't traverse the stub. A type 7 LSA is injected by the ASBR and it stays a type 7 LSA until it reaches the ABR. The ABR then converts it into a type 5 LSA to let the other areas know. Make sense?

lrb

Yes the OCG is correct, Type 5 LSAs are not allowed within the NSSA. If the NSSA ABR was to receive a Type 5 LSA from within the backbone, it would filter it from going into the NSSA (in the same way they would be filtered from entering a stub area)

The ABR that is connected to the NSSA will translate the Type 7 LSA to a Type 5 LSA to advertise the external network into the backbone.

An interesting point about this is that the NSSA ABR actually becomes a kind of pseudo-ASBR because it is generating the Type 5 LSAs. As a result, this ABR:

1. Does not originate the Type 4 LSA listing the ASBR that is generating the Type 5 LSAs (because it is acting a pseudo-ASBR)
2. Can use the summary-address to create a summary route to advertise into the routing domain.

Hope that helps.

hitmen

I still dont understand why type 5 lsa stop injection for all 4 types of network. That is what is confusing me

lrb

You don't need the Type 5 LSAs in stub areas because of the Type 3 LSA originated by the ABR which advertises a default route. You can take this one step further by adding the no-summary keyword to the stub configuration on the ABR meaning that it will only send a single Type 3 LSA into the stub area, which advertises the default route.

If you think about a stub area with a few routers and a single ABR, all of the inter-area and AS external traffic from the endpoints connected to those routers will need to traverse the ABR to leave the area right? This is why routers in the area do not need the Type 5 LSAs - they just need to know how to reach the ABR.