[nsp] always-compare-med

[Dave Wilson]

We're deploying a backup transit service from one of our upstreams, and we 
hit an unexpected issue when trying to use MEDs to choose between the paths.

We have two STM-16 links (capped to STM-4) to upstream A, and a single STM-1 
to upstream B. We only want one of the links to A in use at any time, but 
we'd like to take advantage of B throughout. Inbound traffic is not a problem.

For outbound traffic, we apply identical localpreferences to prefixes 
received from each link, allow AS path length to take its course (which 
distributes traffic evenly between A and B), and set metrics on the prefixes 
we receive from A - 0 on the primary link, and 40 on the backup. However, 
this last step doesn't work, because each prefix is also received via B.

According to the best path selection algorithm, the comparison of MEDs is 
ignored in this circumstance. It looks like we have to turn on 
always-compare-med. As far as I can tell, this won't significantly affect 
traffic to our other peers, because localpref and AS path length are 
evaluated before MEDs are used. Do others on this list use it, and has it 
caused problems?

Also, is there another way to implement the above? It doesn't suffice to 
give ISP B a lower localpref, because its line would remain empty except 
during an outage. We can't give the backup line to A a lower localpref, 
because during failure of the primary line, all traffic would divert to B 
(which is much lower bandwidth). I've thought about doing as-path stuffing 
on *received* routes. It works in the test lab, but is it against the rules 
in real life?

Thanks,
Dave

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