[c-nsp] Peering + transit backup between 2 ISPs

Jon Lewis jlewis at lewis.org
Thu Nov 24 15:20:54 EST 2005

On Thu, 24 Nov 2005, Jon Lewis wrote:

> The really simple way to do this would be for A and B to setup normal eBGP
> (as if providing transit to each other), but to use as-path prepending
> such that when A propogates B's routes to PA (A's provider), those routes
> get several prepends, and the same setup for when B propogates A's routes
> to PB.  The trouble is, that won't really work, because some people (those
> who do BGP tuning by setting localpref) will ignore your as-path lengths,
> and you're likely to end up providing at least a little transit to each
> other all the time.

I realize I should have also explicitly mentioned that in the above setup, 
you'd want to make very sure that you have something in place (whether you 
use communities, as-path access-lists, or something else) to make sure 
that A never propogates B's provider's routes to A's provider and vice 

  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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