[nsp] always-compare-med

[Andrey Kostin]

Hello Dave,
may be this link about conditions of path selection with MED in bgp will
help you:
http://www.cisco.com/warp/public/459/bgp-med.html
-----------------------------------
Andrey Kostin, Demos-Internet ISP
http://www.demos-internet.ru/


>
> 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
>
> --
> Native IPv6 peers: Internet 2
> ---------------------------------------------
> Native IPv6 customers: TCD
> dave.wilson at heanet.ie
> Native IPv6 PoPs: Dublin, Citywest                    tel:
> +353-1-660-9040
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> +353-1-660-3666
>
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