Hello Kevin
Ah yes, that’s a great observation! Let me clarify. 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 does indeed show a full mesh of adjacencies, but 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
I hope this has been helpful!
Laz