North American Network Operators Group

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Dynamically Changing Exit Policy (iBGP)

  • From: Benjamin Howell
  • Date: Mon Oct 29 16:46:40 2007

Is there a generally accepted method of automatically altering exit
policies within an AS?

I'd like to dynamically change from best-exit to a "hot potato" exit
policy when an internal DS3 fails. We fail over to a much lower
bandwidth link and would like to avoid sending anything but internal
traffic over that link. If it's not already clear, this change needs to
happen automatically.

I realize that there are two means of accomplishing this:

(1)  Set a weight on all routes received from the eBGP peer at each
     location so that it prefers the direct eBGP peer.
(2)  Sever the iBGP session by tying the iBGP session to an interface
     IP address rather than a loopback IP. When the DS3 goes down, so
     will the knowledge of the remote exit point.

The devil's in the details however. I can't figure out how to make the
weight approach work on routes that were received prior to the circuit
failure or how to remove the weights once the circuit comes back up.

Severing the iBGP session seems drastic to me, and I'm worried that our
advertised routes will be dampened by peers if the internal DS3 starts

Any input from wiser peers would be greatly appreciated.

Ben Howell