EIGRP offset lists

A simplistic explanation of EIGRP offset-list configuration command you might see every now and then is “it adjusts the RD/FD to influence route selection”. If that would be the case, the adjustment would not be propagated to upstream routers (remember: only the EIGRP vector metric is sent in the routing updates, not RD or FD) resulting in potential routing loops (it’s never a good idea to use one set of metrics and propagate another set of metrics to your neighbors).

In reality, the EIGRP offset lists adjust the delay portion of the EIGRP vector metric (which linearly influences the RD/FD value). You can increase or decrease the value of the delay metric for EIGRP updates received or sent through a specific interface (or all interfaces). You can also use an access list in the offset-list command, applying changes only to specific IP prefixes (very similar to what I described in the Scalable policy routing IP corner article). For more details, please read this technology note on Cisco’s web site.

2 comments:

  1. Hi Ivan,

    Can you really decrease the value of the delay metric for EIGRP updates received or sent ?
    I think I had read the same thing about decreasing the hop count of RIP updates using offset-list but didn't find how to do it, and I am not sure you would want to do something like that. Would that not be a recipe for routing loops ?

    Cheers,
    Mat

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  2. Ivan Pepelnjak06 July, 2010 12:41

    Although the offset-list CLI help claims you can increase or decrease RIP hop count (even if you use it in EIGRP ;) ), you cannot enter negative offset. You can only increase hop count (with RIP) or delay (with EIGRP).
    And, as you’ve said, entering negative offset would be a nice recipe for a routing loop.

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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.