Configuring Hello Packet Parameters

You can set the hello interval and the hello multiplier for IS-IS hello packets.

isis hello-interval

isis hello-multiplier

• Use the isis hello-interval command to set the length of time (in seconds) between hello packets sent on a specific interface. Configure independently for level 1 and level 2, except on point-to-point interfaces because only a single type of hello packet is sent on serial links. For this reason, it is independent of levels 1 and 2. For example, you can specify an optional level for Frame Relay multiaccess networks.

  The hello-interval is equal to the hello multiplier times the hello interval seconds and is advertised as the holdtime in the hello packets transmitted. The range is 0–65535; the default value is 10 seconds.

  Note: The hello-interval value must be the same for all routers attached to a common network. With smaller hello intervals, topological changes are detected faster, but there is more routing traffic.

• Use the isis hello-multiplier command to set a number by which to multiply the hello interval seconds. This number determines the total holding time transmitted in the IS-IS hello packet. The default is 3. Use when hello packets are frequently lost and IS-IS adjacencies are failing unnecessarily.

  The advertised hold time in IS-IS hellos is set to the hello-multiplier times the hello-interval. Neighbors declare an adjacency to this router to be down after not having received any IS-IS hellos during the advertised hold time.

  • The hold time (and thus the hello-multiplier and the hello-interval) can be set on a per interface basis, and can be different between different routers in one area.
  • Using a smaller hello-multiplier will give fast convergence, but can result in more routing instability.
  • Increment the hello-multiplier to a larger value to help network stability when needed.

    Caution: Never configure a hello-multiplier lower than the default.

• Holding time—Time a neighbor waits for another hello packet before declaring the neighbor is down. It determines how quickly a failed link or neighbor is identified so that routes can be recalculated.
• Raise the hello multiplier and lower the hello interval simultaneously to make the hello protocol more reliable without increasing the time required to detect a link failure.
• Example

  host1(config-if)#isis hello-interval 6 level-1

  host1(config-if)#isis hello-multiplier 10 level-1

• Use the no version to restore a default value.
• See isis hello-interval
• See isis hello-multiplier

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