BGP load sharing unequal paths - how would you do it?

Discussion in 'Cisco' started by Rob, Feb 21, 2004.

  1. Rob

    Rob Guest

    Right now I have a 7204 load sharing four T1's. Two from MCI and two
    from Global Crossing. We run BGP and announce a class C range that we
    own, over both ISP's. We get full routing tables from both.

    It has given us pretty even load-sharing considering both MCI and GBX
    are Tier 1 providers. This method has been fine for years but it is
    about to change.

    The Global Crossing connection is going up to a partial DS3 (18Mb
    woohoo!), while the MCI will remain at 3Mb total for now. It may go
    up to 3 or 4 T1's later, but right now the ratio is 3Mb to 18Mb.

    All Internet routes need to be available to both ISP's, so failover
    and redundancy are important. I don't want to limit the BGP updates I
    get from MCI to balance the links.

    What is the best way to load-share disproportionate links? If at all
    possible, I would like most of the traffic to go over the Global
    Crossing T3 with only the closest ASN's getting time on MCI. And if
    the Global Crossing DS3 took a dive, I'd want everything to route over
    MCI until it was restored.

    What is the best practice being used today to solve this question?

    Or would it be better just to treat the MCI circuits as a pure backup
    since in 95% of the instances, the Global circuit would be faster?

    Rob, Feb 21, 2004
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  2. Rob

    Tim Thorne Guest

    I'd keep prepending your AS onto your MCI advertisements until you've
    reached an acceptable traffic split. If Gblx takes a dive, then the
    MCI line will take the strain until it comes back. Use a route map and
    set the prepend in there.

    If you're going to utilise your 18MB cct fully, then failing over to a
    3MB cct will bring along problems of its own. You'll flood the MCI cct
    badly and latency will rise horribly. I'd sit down and think about the
    traffic levels and types of traffic you need to pass over the MCI cct
    during a Gblx outage very carefully.

    Tim Thorne, Feb 21, 2004
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  3. Rob

    Rob Guest

    Thanks Tim.
    But doesn't prepending the AS affect traffic in only one direction?

    Yes, I know that sending 18Mb through a 3Mb pipe would be bad, but
    it's better than nothing. We're also contemplating bumping that up to
    a MLPPP bundle of four T1's.
    Rob, Feb 21, 2004
  4. Rob

    Tim Thorne Guest

    You can tag inbound routes with AS path prepends to engineer your
    outbound traffic. I dislike leaving links idle. If something happens
    to your announcements over that link you'll never see the problem
    until you need to utilise the link. Your quickest and easiest solution
    would be the one you've mentioned, use the MCI link purely as backup.

    Tim Thorne, Feb 21, 2004
  5. Regardless of the approach you take, if higher availability is desired,
    it is essential that you monitor the state of both links so repairs can
    be initiated before the second link fails. Unless you routinely monitor
    your BGP neighbor tables (does anyone?) or otherwise have a dedicated
    monitor (such as WhatsUp Gold against the IP of the other end of the
    link), the easiest way to detect a failure is to keep an eye on your
    MRTG or other traffic monitor, use load sharing, and jump into action
    any time you see one of the links drop to zero traffic.

    Good luck and have fun!
    Vincent C Jones, Feb 21, 2004
  6. I'd either treat the MCI circuit purely as a backup, or accept only
    partial routes and a default from MCI (i.e., MCI & their direct
    customers only), which will cause everything else to be sent via GBX.
    If you're really opposed to getting customer-only routes from MCI, get
    them all and use a route-map to raise the local preference of routes
    that only have AS701 in the AS-PATH, or ask MCI if they can send you
    all routes but tag customer routes with a community which you can use
    to differentiate them.

    Terry Baranski, Feb 21, 2004
  7. Rob

    Rob Guest

    That sounds interesting. Is there a writeup on CCO?

    Rob, Feb 21, 2004
    may help, though the book "Internet Routing Architectures" should be
    considered required reading for those who work with BGP.

    I forgot to mention that you'll also want to manipulate your outbound
    advertisement to MCI in some way so that only MCI and it's direct
    customers use your MCI link to get traffic to you. Best to talk to
    MCI and see what they recommend, as they may have a specific community
    you can tag your advertisement with to accomplish this based on GBX
    (probably) being a peer of theirs. If not, prepending is probably the
    way to go.

    Terry Baranski, Feb 21, 2004
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