Recommendations for keepalive/hello timers

The “GRE keepalives or EIGRP hellos” discussion has triggered another interesting question:

Is there a good rule-of-thumb for setting hold-down timers in respect to the bandwidth/delay of a given link? Perhaps something based off of the SRTT?

Routing protocol hello packets or GRE keepalive packets are small compared to the bandwidths we have today and common RTT values are measured in milliseconds while the timers' granularity is usually in seconds.

OSPF and IS-IS support for fast hellos is an exception, but you wouldn’t want to use this feature on a hub router with tens or hundreds of small remote sites.

You should answer the above question by asking yourself: what are my business needs for a fast switchover and how can I get there? If you’re satisfied with a switchover that takes a few (up to ten) seconds, you can achieve it with keepalive/hello packets. If you need a faster switchover, you will have to do serious routing protocol tuning or use MPLS TE fast reroute.

3 comments:

  1. Hi Ivan,

    You should add to your note ISIS capability for supporting fast hellos.

    ReplyDelete
  2. You're absolutely correct. Fixed the post. Thanks.

    ReplyDelete
  3. Along these lines, is anyone experiencing issues with OSPF fast-hellos in a campus environment with Cisco Voice?

    ReplyDelete

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.