[nsp] BGP question

[Mark Tinka]

cisco-nsp-bounces at puck.nether.net wrote:
> I'm still a newbie at bgp, so please bear with me.
> I have two links to the internet.
> A lot of routes from both providers have equal AS paths.
> If one bgp session gets reset all that traffic (the equal AS path
> routes) goes out to provider2 and does'nt reset back to provider1,
> providing that the bgp session to provider1 was the first one to come
> up. I don't have any metrics, weights or localpref on any routes. Is
> there any way to have those equal path routes default to provider1
> rather than provider2 if provider1's link and bgp session are up?
> Right now in order to reset those routes back to provider1 I have to
> do a hard "clear ip bgp provider2" reset.      
> 
> Kevin,
> Honeycomb Internet services

Hi Kevin.

You can use communities, with your ISP, to influence the path selection
process. But even easier, you can make the AS path over the less preferred
ISP, longer, by prepending it along with your BGP announcements to their BGP
speaker.

Basically, the path with the shortest AS path, wins. So, by making the AS
path over ISP 2 longer, you automatically ensure it's only used for return
traffic when ISP 1 is totally unavailable. You can do this by:

route-map PREPEND-ISP-2 permit 10
 set as-path prepend xxx xxx xxx xxx

Where xxx = your own ASN. The more times you prepend your ASN, the longer
the path over ISP 2 appears. IIRC, Cisco route maps will support a maximum
of 10 prepends + 1 which is sent along by default. 

You will then need to apply this route map to your outbound policies to ISP
2, and you're in business.

Regards,

Mark Tinka - CCNP
Network Engineer, Africa Online Uganda