LSA question

Bosefus

Had a quick question about LSA's.

From my understanding Type 1 LSA's contain a complete list of directly connected neighbors.

Assume in the following example that I have 4 router connected via serial links in a straight line with an extra cable connecting Router a to Router D, all in the same area.

(Router A)

---

(Router

---

(Router C)

---

(Router D)

---

When the link between Router A and Router B goes down, Router A sends out an LSA to all interfaces in the area. Routers that receive the LSA (Router D in this example) flood out LSA's with the updates out all of there interfaces in the area, from my understanding, the LSA type being flooded by router A would be type 1 because a directly connected neighbor went down. But what type of LSA would Router D flood?

If type 1 is only directly connected neigbors, router D should not be flooding type 1's because a link to a directly connected neighbor has not gone down.

Thanks

EdTheLad

You are misunderstanding how type 1 LSAs are generated and exchanged.

Each router creates a type 1 LSA by looking at its directly connected links.When two routers form an adjacency they exchange type 1 LSAs, they dont just exchange their locally generated LSA, they also exchange LSAs received by other adjacent neighbors.Every router in an area will have an identical list of type LSAs, using these LSAs a database is created which will also be identical.Then each router putting itself as the root of the tree will perform a SPT calculation to determine the cost to each destination.

Check out the ospf rfc for more info.

Bosefus

Thanks for the reply, you have cleared up my misunderstanding.

The books definition (Cisco Press Exam Guide) of type 1 LSA's made them seem like they only ever contained locally connected neighbors.

kryolla

Bosefus wrote: »

Thanks for the reply, you have cleared up my misunderstanding.

The books definition (Cisco Press Exam Guide) of type 1 LSA's made them seem like they only ever contained locally connected neighbors.

This is true. If you look at the router LSA all they contain are their directly connected links. But you also have LSA flooding which is propagating all the router LSA to every router in the area so every router has the exact link state database.

R1---R2---R3

R2 and R3 have formed an adjacency and all is good. Now R1 is added to the network and will exchange its router LSAs to R2 and R2 will flood this to R3 and at the same time R2 will flood all the LSA it knows about to R1 except the LSA it received from R1. Now all 3 routers have the same link state database and will perform SPF with itself being the root and the network will be converged.

networker050184

Bosefus wrote: »

Thanks for the reply, you have cleared up my misunderstanding.

The books definition (Cisco Press Exam Guide) of type 1 LSA's made them seem like they only ever contained locally connected neighbors.

Just one point I'm not sure if you are getting. The type 1 LSA will have directly connected links as others have pointed out, not only locally connected neighbors.

Bosefus

Thank you for clearing that up for me.