[c-nsp] BGP Multihomed Selective/Conditional Advertisement

[tkacprzynski at SpencerStuart.com]

I have been trying to figure out how to do this and maybe someone will be able to help me out.
 
I have two ISP connections ISP ATT and ISP Cogent. 
 
(ISP Cogent)    (ISP ATT)
     |                       |
  RO ------------------- R1
 
 
ATT would be used for primarily internet and access to our webservers.
 
Cogent would be primarily used to access Cognet's network that use VPN for incoming connections only. I do not want to have other networks besides Cogent's network using this path to access our webserver.
 
I would like to have each other act as a backup for one another. For instance if ATT fails I want everyone on the internet use Cogent to access me. If Cogent fails I want everyone on the internet and the VPN connections on Cogent's network to use ATT.
 
So basically what I was thinking to setup is to accept a default router from ATT and Cogent. Lower the local preference of Cogent and that way I would accomplish using ATT as primary internet access.
 
The tricky part is with Cogent and using then to only access their local networks. Looking through communities I found out Cogent's communities that would not export my route to their peers and keep it internal within their AS. This works fine but the problem now is how do I failover if ATT fails? How do I automatically change my not-export community I'm sending to Cogent to start adverting the route to its peers?
 
I looked at conditional advertisement, I was able to basically send the route map with not-export communities to Cogent if the default route from ATT is present. The problem with this is that once the default router disappears it doesn't advertise anything to Cogent, none of my routes are advertised to Cogent.
 
I'm not sure if I could do this sort of a double condition such as 
 
if ATT's default route is present send out to Cogent a route map with prefixes to not-export my routes
if ATT's default route is not present sent to Cogent a route map without any communities on my routes
 
Basically I'm trying to figure out how I can have multihoming, but with the constrains that I want 1 ISP to be used for internet and the other to only access their AS, but still have the capability to automatically failover in case one of the circuits dies.
 
Thank you for any input or help.
 
 
Tom Kacprzyński
Network Engineer