Load sharing between links where AS path length is equal

Discussion in 'Cisco' started by Andrew Gideon, Feb 27, 2005.

  1. For the first time I've two decent links, to two different ASs, on a single
    router. For a large portion of the routes received from both upstreams,
    path length is identical.

    The discriminating factor on this router between the two paths from the two
    different upstreams to the same destination AS has become "which path was
    received first". This is tending to send most traffic through one of the
    upstreams, unfortunately, leaving the other path underused.

    How is this best corrected so both links are used [roughly] equally?

    I'm thinking that "multipath" might be a solution. But from reading:


    I see something of a "red flag". Under the "BGP Multicast" heading, this
    document claims that "To be candidates for multipath, paths to the same
    destination need to have the following characteristics equal to the best
    path's characteristics:" and included in this list are: "Neighboring AS or
    sub-AS" and "AS-PATH". So Multipath really seems oriented towards multiple
    circuits into the same AS.

    Since my two upstreams have two different ASNs, this would seem to preclude
    use of Multipath to load balance.

    Another possibility is "bgp bestpath as-path ignore", but this would seem a
    Bad Thing for those less common cases where there is a difference in path
    lengths between the two circuits. Besides, I suspect that this would
    merely cause *all* choices to be based upon the age of the received path
    which would increase, and not decrease, my imbalance.

    I see plenty of ways to force choices between one link or the other, but I'm
    missing a way to more equally share the load. Suggestions?


    Andrew Gideon, Feb 27, 2005
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  2. Andrew Gideon

    Merv Guest

    stay away from multipath unless you want to blow up your router.

    one of the tradional ways to lokad balance two T1 to differnet AS is:

    1. use AS prepend on outbound advertisements of your space -splitting
    your spce into two. Require that you have sufficent address space to do

    2. local prefer some of the "big bous" AS number; prefer some on link1
    versus some others on like 2.

    ISP AS #
    === ====
    Qwest 209
    UUNet 701
    Sprint 1239
    C&W 3561
    Level3 3356
    AT&T 7018
    Genuity/BBN 1
    Verio 2914

    If you are going to do much BGP work, I highly recommend that you get
    the book
    "Internet Routing Architectures" by Sam Halabi
    Merv, Feb 27, 2005
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  3. That's it? I'd used that idea myself once when I'd very asymmetric
    connectivity on one router. But I've always presumed that, for the
    symmetric case, there'd be a cleaner solution.

    Is this case really that uncommon? I cannot imagine so. What am I missing

    BTW, thanks for the book reference. I'll get/read it.

    - Andrew
    Andrew Gideon, Mar 1, 2005
  4. Andrew Gideon

    Merv Guest

    The case in question is quite common. And the solution I suggested is
    the commonly used to address it; I am not aware of any magic bullet ..
    Merv, Mar 1, 2005
  5. But it's kind of a "hack", no? I mean, a significant change in network
    topology (ie. one large AS consuming another, like Level3 buying BBN) could
    easily change the traffic balance. That seems...fragile (which is a common
    consequence of a "hack" in my mind {8^).

    Given how common this is, I'd expect there to be - as you put it - a "magic
    buttet": Some way to (roughly) balance traffic in the same way two equally
    weighted static routes would (roughly) balance traffic between two paths.

    I do feel like I'm missing something, even if only a reason why this would
    be a Bad Thing.

    - Andrew
    Andrew Gideon, Mar 1, 2005
  6. Hmm. Perhaps I'm not missing anything, even though I find this rather
    disappointing and not a little surprising.



    - Andrew
    Andrew Gideon, Mar 1, 2005
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