IS-IS: Operation over Broadcast Links

IS-IS routers must create adjacencies, sync their databases, and keep them synchronized as usual.  Nothing different here.  However IS-IS packets for Level 1 are sent to multicast address 0180.c200.0014.  Level 2 packets are sent to multicast address 0180.c200.0015.

IIHs are still used to discover neighbors and they will include the SNPA of all neighbors the originating router receives IIHs from.  When routers receive these IIHs and see their own SNPA they know to turn their adjacency all the way up.

IS-IS will also determine a DIS over a multiaccess network.  The DIS is elected based off the following criteria:

  • the router with the highest interface priority
  • If there is a tie, the router with the highest SNPA
  • In case SNPAs are not comparable, the router with the highest System ID.  This rule is for Frame Relay and ATM networks as IS-IS treats these interfaces as broadcast interfaces.

One thing to keep in mind about DIS elections is that IS-IS routers are preemptive and will take over the role of DIS if a new routers priority or SPNA or System ID is highest.

A DIS is responsible for two things:

  • Helping routers on a broadcast segment synchronize
  • Representing the broadcast segment in the LSDB as a standalone object or Pseudonode

The pseudonode will send CSNPs on a very short interval every ten seconds.  IS-IS routers process these CSNPs as they would normally do from a regular IS-IS neighbor.  PSNPs will be sent for any unknown or outdated LSPIDs as usual to the DIS and the DIS would flood those requested LSPs as needed.

The Pseudonode must have its own database with each neighbor in order for path selection to work.  To do this the DIS must send a unique LSP for the DIS out to the network on behalf of the pseudonode.

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