[c-nsp] BGP outbound loadsharing

Justin M. Streiner streiner at cluebyfour.org
Fri Jan 23 13:23:41 EST 2009

On Fri, 23 Jan 2009, Kevin Loch wrote:

>>  For example, if you were downloading a full BGP table from both ISPs and
>>  assigned local preference to some routes, traffic from a customer could
>>  arrive on ISP1 and return traffic be sent via ISP2. Is this generally
>>  considered to be acceptable or is it preferential for return traffic to be
>>  routed the same ISP from which it came into your network?
> It is completely normal and common for traffic to take different paths in and 
> out especially when both endpoints have different paths
> available.  There is no reason to be concerned about this from a
> technical perspective.

I'll second this.  Asymmetric paths are a normal occurrence.  There is a 
common misconception that asymmetry is bad.  Granted it can make 
troubleshooting a little more involved at times, but it is not a bad 
thing.  Plus, once you hand traffic off to another network, and even 
moreso when they hand of off to another network that is not connected to 
yours at all, you don't really have much control over the path to the 

Your upstreams might let you tag routes you announce to them with 
specific BGP communities to have some influence over what they do 
with the traffic, but things like local-preference and MEDs are not 
transitive attributes, i.e. they are not propagated past your upstreams. 
The only transitive attribute you can generally modify is the AS path.


More information about the cisco-nsp mailing list