[c-nsp] BGP Multihomed Selective/Conditional Advertisement

[Seth Mattinen]

tkacprzynski at SpencerStuart.com wrote:
> Nathan, thanks for this idea. Your idea could work. I just need to find
> out if they will accept my 2x /25  routes if I split the /24. 
> 
> As for the prepend, how could I deal with peers that might be closer to
> Cogent than ATT and not load-balance that traffic based on peer's
> peering location?
> 
> The usage of Cogent is primarily for point to point VPN backup from
> other offices that would be on Cogent's network. Ideally I would like to
> keep that link free of internet traffic unless the primary (ATT) link
> fails and also not have asymmetric traffic where traffic comes in on the
> Cogent circuit and leaves on ATT's circuit because of the default route
> match. I'm not sure if that's possible, but that would be my ideal
> solution.
> 

Use BGP communities. Tag your announcement with a community that will 
set the localpref for your route within Cogent as low as possible, that 
should do the trick and you shouldn't see any traffic from them unless 
your other announcements are withdrawn. (Do the same internally for 
routes learned via your backup link.) This works better than trying to 
announce a /25 that the rest of the internet is unlikely to honor, and 
without weird BGP tricks. Internally, you'll have to add route maps or 
something to bump the localpref to prefer links to your backup offices.

As for peers closer to Cogent, if you localpref it extremely low to 
Cogent and the ATT routes are higher, these peers (even within Cogent's 
network) will no longer see Cogent as preferred. I do this with my 
neighbors when I'm about to perform maintenance on one of the border 
routers.

I haven't read the rest of the thread, so this advice is just based on 
the preceding message. You'll probably have to mix and match this advice 
with others. ;)

~Seth