IS-IS DIS and Pseudonode

This topic is to discuss the following lesson:

Hi Rene,
Is there any way to make it P2P like OSPF ? Thanks


Hello Zaman,

There is yes, you can use this command:

R1(config-if)#isis network ?
  point-to-point  Specify ISIS point-to-point network

You can use this on broadcast networks (like Ethernet) when you only have two routers. This disables the creation/election of a DIS so it’s more efficient.


Hello Rene
i have Problem with understanding that DIS / PSN
please explain it to me more simple

Which is far less than the six neighbor adjacencies we would have without the pseudonode where each router would report three neighbor adjacencies.
As per this sentence there are only 3 neighbor adjacencies. But when we configured ISIS each routers have full mesh Neighbourship with each other .I am still confuse how does DIS help to reduce the neighbor adjacencies ?

Thanks in advance

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!


Another issue is that the link-state database can grow exponentially. Can you explain a bit more about it? How LSD may grow without Pseudonode. As Each router will send the LSP and others are going except it. Eventually, all routers receive the LSP from each router in LAN. In that case how come link-state database got reduced with pseudonode