How to configure backup route with BGP

Discussion in 'Cisco' started by Michel DUBOIS, Jul 1, 2004.

  1. Hello,
    My Cisco 7200 is connected with two different providers ... and BGP is
    enable with both. How can i can configure the second link only to be
    used when the other went down ?
    Thank you ...
    Michel DUBOIS, Jul 1, 2004
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  2. Michel DUBOIS

    Ivan Ostres Guest

    You can use AS prepend for both incoming and outgoing routes for
    sepcific ISP. Or AS prependig for outgoing routes and weight or local
    pref for incoming routes.

    Ivan Ostres, Jul 1, 2004
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  3. It doesn't guarantee that the second link will sit idle when the first
    is working. Backup ISP could still prefer direct route to their customer
    instead of sending traffic via scenic path.

    Alternative apporaches are:
    1) Conditional advertising. It does guarantee then backup will be
    2) Use communities (if ISP supports customer's communities) to signal
    lower localpref to second ISP. It works better than prepending but still
    doesn't guarantee idleness of the backup link.
    3) Deaggregation. Split your netblock in two. Announce aggregate and
    both halves via primary ISP but only aggregate via backup. It still
    doesn't ensure that backup link will not be used and it is frowned upon
    by internet community.
    Daniel Ginsburg, Jul 1, 2004
  4. There are multiple ways to accomplish this, but the important thing to
    remember is that incoming packets and outbound packets are independently
    routed and you need to configure BGP to control both directions. You
    also need to decide if you want traffic to and from users served by the
    backup ISP to use the backup path or take the long route through your
    primary ISP.

    Which technique is best for outbound depends upon how you are using
    BGP. Are you accepting full routes from each ISP, local routes only,
    indicator routes, or just a default route.

    Which technique is best for inbound will depend upon your backup ISP.
    Many ISPs support this mode of operation directly by defining a
    community string which classifies advertisements as local or global
    backup use only. Ask your backup ISP what they recommend. If they don't
    provide explicit support, then you can always use AS path prepending,
    but that makes it difficult to use your backup link for users local to
    the backup ISP.

    BGP is like a Swiss Army knife, and provides multiple ways of
    implementing routing policies. You need to decide what you want your
    routing policies to be, then pick the best mechanism based on what your
    ISPs will support and how they are connected.

    Good luck and have fun!
    Vincent C Jones, Jul 1, 2004
  5. Michel DUBOIS


    Jun 19, 2009
    Likes Received:
    Last edited: Sep 16, 2009
    theapplebee, Sep 8, 2009
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