[nsp] Balancing traffic with unequal AS paths

[Pete Templin]

AS path prepending is a moderate stick, less brutal than local 
preference or weight, but nonetheless still a bit aggressive in some 
situations.

I would begin by researching the list of common AS between A and C.  For 
those AS paths, I would prepend "A A" or "A A A" on routes learned from 
A to make them less preferable.  Likewise, assuming you want to do the 
relatively same thing on inbound traffic, I'd prepend "U U" or "U U U" 
on your announcements to A.  ("U" means YOU in this case.  :)

After this is done, you get the joy of balancing the ASes that are not 
common to A and C.  I'd recommend starting with one prepend initially on 
whichever side you want to decrease your traffic, and see how that works.

Sam Stickland wrote:
> Hi,
> 
> We're receiving partial IX feeds from two providers. One of these providers
> is recieving it's partial feed through another AS (a company it recently
> absorbed). So we have this situation
> 
>      C
>      |
>      B      A
>      |      |
>   ------Us------
> 
> Feed B-C is from an IX fairly close to us, feed B is from an IX further
> away. A lot of people have presences at both of these exchanges, so when
> choosing our outbound traffic we want to prefer C-B over A. (Otherwise
> traffic that should be dumped locally can end up getting bounced off of the
> remote IX).
> 
> Ideally if the AS paths were equal length they would both have the same
> local preference, but routes from B would be given a lower MED. But the AS
> paths lengths aren't equal (even though the addition of AS C doesn't affect
> performance), so using the same local-pref will send traffic via A.
> 
> How can I get the traffic distribution that I want? I've heard people treat
> inbound AS path prefixing like a boogey man before - is that jusified, or it
> is the solution in this case?
> 
> Sam
> 
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