IS-IS Designated Intermediate System


This document is about the link-state packet (LSP) pseudonode. A pseudonode is a logical representation of the LAN which is generated by a Designated Intermediate System (DIS) on a LAN segment. DIS is similar to OSPF's DR.


  • DIS is similar to OSPF's DR, but there is no BDR.
  • Pseudonodes is created for a broadcast segment to represent its all devices connected on that link. It reduces the number of adj that have to be maintained.
  • Main functionality is to:
    • Create and update the pseudonode Link-State Packet (LSP) (it is a separate one from the router's LSP).
    • Flood LSPs over the LAN. Flooding complete sequence number protocol data units (CSNPs) every 10 seconds.
  • Different DIS can exist for Level1 and Level2.
  • How to detect which devices is the pseudonode? The router with metric 0 to all attached routers will be the pseudonode. All other devices will have a non-zero cost.
  • The devices elected as DIS will generate a separate Link-State Packet (LSP) for the pseudonode, with metric 0 to all other routers. That same router will generate a LSP for itself with a cost to the pseudonode (see example below).

DIS Election

  • Election is done based on:
    • 1. Highest Priority
    • 2. Highest SNPA (MAC) address.
  • Separate election is held for DIS Level 1 and DIS Level 2.
  • DIS priority 0 is the lowest priority, but could still become the DIS, unlike OSPF.
  • Default priority is 64.
  • No Backup DIS, new one is elected upon primary failure.
  • DIS election is preemptive.

Additional Resources

Understanding IS-IS Pseudonode LSP


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