IS-IS Pseudonode and DIS (Designated IS)

In IS-IS a broadcast link itself is modeled as a pseudo-node that connects all attached routers to a star topology.
The pseudo-node is represented by the DIS (Designated router)



On broadcast type links a DIS is elected for both Level-1 and Level-2, if they exist.
A router with highest priority or highest SNPA (MAC,DLCI,..) address is selected as DIS.
Default priority is 64. (can be between 0-127)
DIS election can be preempted, any priority change will take effect immediately.

There is no backup DIS (unlike OSPF) and all routers in a LAN establish adjacencies with each other and the DIS.

The DIS creates and maintains the pseudo-node LSP.
This LSP contains the adjacent ISs, just like the network lsa in OSPF.