IS IS Broadcast


IS-IS has network types similar to OSPF. This page describes the behavior of broadcast circuit type.

ISIS Broadcast

  • Circuit type used on multiaccess interfaces like Ethernet.
  • Within a broadcast network, a Designated Intermediate System is elected (DIS).

LSP Flooding

  • DIS performs flooding of LSA updates to a multicast address. That address is different depending on the Level:
    • 01-80-C2-00-00-15 for Level 2 adjacencies (AllL2ISs)
    • 01-80-C2-00-00-14 for Level 1 adjacencies (AllL1ISs)
  • Flooded LSP are not ACK in a broadcast network. The DIS periodically sends out the CNSP (Complete) update. By default it's 10 seconds and can be changed using the command isis csnp-interval.

DIS Election

  • Election is done based on:
    • Highest Priority
    • 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.
  • No Backup DIS, new one is elected upon primary failure.
  • DIS election is preemptive.

Relevant IOS Commands

isis csnp-interval

On broadcast networks, advertising interval of complete sequence number PDUs.

R1(config-if)#        isis csnp-interval (sec)


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