AS path question.

Jack Bates jbates at
Wed Nov 10 15:37:05 CST 2010

On 11/10/2010 2:44 PM, Stefan Fouant wrote:
> In this day and age, I'm still surprised at how many people think AS Path
> prepending is the preferred way to influence inbound traffic paths.  As many
> providers have default local-pref policies in place which prefer routes
> learned from the customer over routes learned from another peer, it is often
> the case that a supposed secondary path is actual the preferred path from
> the view of that upstream provider.

It depends on what you want. In many cases, route preference only 
extends one AS. It is rare that those AS also give you options for 
influencing preferences at their next peer. Given that, a prepend often 
is used to force people to generally chose one path over another, while 
still allowing traffic in on the secondary peer for any traffic that had 
to hit it anyways. This has the bonus of also making sure you don't send 
traffic the long way around when it does hit the neighbor AS. I 
generally don't prepend more than 3-4 times, though.

> IMO, a combination of both community tagging to influence localpref coupled
> with AS Path prepending on the secondary link is the best approach, and
> seems to accommodate both steady state as well as failure scenarios
> properly.

There is no proof in this case that all of these things weren't done. We 
only have the evidence of the Path prepend. Of course, the default 
localpref for most of my transits is to prefer me over other peerings, 
so I rarely adjust that unless I don't want any traffic what so ever.


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