Multihomed BGP on one router causing memory probs?

Discussion in 'Cisco' started by Dan Berlin, Jun 18, 2004.

  1. Dan Berlin

    Dan Berlin Guest

    I have a multihomed BGP setup to two different ISPs. It is running on
    my cisco 3640, which gets the full BGP routing tables. Lately, the
    router has been having serious memory problems due to the size of the
    BGP tables, and I would like to find a way to setup the following, if
    Use ISP A for all traffic, and only download partial tables from ISP
    A. If ISP A fails, then bring up ISP B and use that. I would like to
    do this temporarily on one router until we can get a second one. Is
    this possible? I've tried to conjur up any way to handle this, but in
    all cases it seems that I need to have the full BGP tables from both
    ISPs because I need both interfaces up.

    I have one solution that I do NOT want to implement, because it seems
    like a very poor solution, but it's all I can think of: issue a
    shutdown to ISP B's serial interface. Manually monitor the connection
    to ISP A, and if it fails, manually shutdown ISP A and no shut ISP B.
    I could even have a program do this for me from within my network, but
    again, this seems like a poor solution when I have a nice Cisco router
    that seemingly should be able to handle doing this for me... maybe

    ANY help is GREATLY appreciated, since my company's internet drops out
    about 5 times a day due to the router reloading from memory problems.

    PS: I've got all the ram I can throw in this thing, and we can't
    afford to buy totally new routers (nor think we need to)
    Dan Berlin, Jun 18, 2004
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  2. Hi,

    Any reason you need the full routing tables from your isps? Since you can
    live with shutting down one of the interfaces, it seems to me the only thing
    you're interested in to be always connected to the internet. If this is the
    case, setup BGP to only get a default route from your provider. If one fails
    the second will still be there and you'll be fine. Also, 2 default routes
    will certainly not give you any memory problems.
    If you also planned on doing some kind of load balancing, try to get partial
    routes from both providers, but not the full routing table.

    Erik Tamminga, Jun 18, 2004
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  3. Dan Berlin

    Toby Guest

    Their is obviously some specific reason that you are loading the full
    Internet routing table in the first place. Can you advise the group to your
    reason behind this and then we may be able to help in a way that achieves
    your goal.

    The only reason I can think of is that when certain routes are available
    from one ISP only then this is the ISP you would use for that particular
    path. In which case I feel you will need to cough up and upgrade.

    Toby, Jun 18, 2004
  4. There are numerous alternatives to running defaultless with BGP.
    Depending upon the ISPs, you may find it effective to just take
    local routes from each and load share the default route between
    the two (but be careful, because some ISPs have huge local tables).

    My favorite approach is to set my default route to a few key prefixes
    upstream of both providers. That way, even if the POP stays up,
    I'm protected against a ISP which gets disconnected from the rest
    of the world.

    Since you feel you can live with just one link working at a time,
    your solutions can be much more flexible. Getting reliable failover
    and load balancing at the same time can be tricky.

    There is a brief white paper on my website which describes some of
    the many options for multihoming with less than monster routers.

    Good luck and have fun!
    Vincent C Jones, Jun 18, 2004
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