IS-IS - DIS and Pseudonode

IS-IS is a link state routing protocol and requires that all routers in the same area have a synchronized link-state database. When a router floods its LSP carrying its prefixes, it’s important that all routers that receive it somehow acknowledge this. This introduces a problem on a multi-access networks segment like a LAN.

When implementing OSPF, if there are many routers connected to the same broadcast domain, a DR and BDR election takes place. All routers in the broadcast domain create two neighbor adjacencies: One to the DR and one to the BDR. There is also an adjacency between the DR and BDR.

When implementing IS-IS, there is a similar mechanism. The Designated IS (DIS) is elected and it creates a virtual router called a pseudonode. This pseudonode plays the corresponding role of the DR in OSPF. However, the DIS and pseudonode operate differently compared to the DR and BDR of OSPF.

The purpose of the DIS and pseudonode is to reduce the number of adjacencies in the flooding of LSDB updates, not the actual discovery of neighbors or forming of relationships.

The output of the show isis database in such a topology will show a full mesh of adjacencies between all IS-IS routers in a broadcast domain. However, these adjacencies weren’t discovered using individual exchanges of updates between those neighbors. They were established with the exchange of updates between each individual IS-IS router and the pseudonode alone. This reduces the number of exchanged packets.

So even though routers discover each other, full adjacencies (for the purpose of database synchronization) on broadcast networks are only formed with the pseudonode (DIS). This is where the efficiency comes in – rather than each router having a full adjacency with every other router, they all just have one with the pseudonode. This minimizes the LSDB update flooding.

Unlike OSPF, IS-IS makes the distinction between:

  • adjacencies for the purpose of database synchronization
  • and IS-IS neighbor adjacencies