[nsp] Balancing traffic with unequal AS paths

[Sam Stickland]

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