IS-IS DIS and Pseudonode

Lessons Discussion
6

Hello BGP

With the pseudonode, there are a total of four adjacencies in the whole topology. If there were no pseudonode, each router would have to create an adjacency with every other router. That means:

  1. R1 - R2
  2. R1 - R3
  3. R1 - R4
  4. R2 - R3
  5. R2 - R4
  6. R3 - R4

So by using the pseudonode you reduce the number of adjacencies, you reduce the complexity of the topology, and you reduce the number of updates that have to be exchanged between particular routers.

Actually, as per the sentence, each router reports three adjacencies. The total number of adjacencies or unique pairs of routers in the list above is six.

The idea is similar to the use of the DR and BDR in OSPF. In order to appreciate this even more, let’s take an example of 10 routers. WIth 10 routers using a pseduonode, you have 10 adjacencies. Without a pseudonode, if you do the calculations you’ll find you will have 45 adjacencies!

I hope this has been helpful!

Laz