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

[Jon Lewis]

On Thu, 24 Nov 2005, Vincent De Keyzer wrote:

> let ISP A and ISP B, which both have routers in a given room. They are good
> friends and they would like to achieve the following:
>
> 1.	set-up a peering to exchange own AS routes
> 2.	serve as backup transit one for the other: each ISP would have one
> upstream only, and when the unique upstream goes down, it would then (and
> only then) use the other ISP for transit

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.

If you want something fancier, do a cio search for bgp conditional. I've 
never tried it, but I suspect you could use bgp conditional advertisement 
to each only propogate the other's routes to each of your providers if you 
don't see your friend's as-path for one of their routes via your own 
provider.  Take a look at the final config example on 
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml


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