Introduction

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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