[nsp] BGP question

[Mark Tinka]

Sorin CONSTANTINESCU wrote:
> Hi,
> 
> Kevin wanted his egress traffic (upload) to go through preferred
> upstream, not his ingress (download) traffic. 

Well then, in that case, even easier; simply modify the LOCAL_PREF attribute
of the routes learned from ISP 2, and set that value lower.

Cisco defaults to 100, so you can set local preference for routes learned
from ISP 2 [either default information, or a full BGP feed] to something
like 80. The routes with a higher local preference, win, and get installed
from the BGP Loc-RIB into the router's active routing table. 

You can do that with:

route-map bgp-feed-in permit 10
 set local-preference 80

Then apply this to the inbound annoucements coming from ISP 2, the less
prefered circuit. I'd recommend using the Route Refresh capability to effect
the new policies :-).

> 
> Mark Tinka said:
>> 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
>> 
>> 
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Regards,

Mark Tinka - CCNP
Network Engineer, Africa Online Uganda