BIRD: route selection question

Martin Barry marty at
Fri May 13 07:21:57 CEST 2011

Apologies for replying to a really old message but...

$quoted_author = "Ondrej Zajicek" ;
> On Mon, Mar 28, 2011 at 03:25:51PM +0400, Mikhail A. Grishin wrote:
> > Hi, Ondrej
> >
> > Thank you for explanation.
> >
> > One more question: is it possible to add "bgp always-compare-med" option  
> > in BIRD?
> That would not be a problem. A question is under which circumstances such
> comparison has some sense. Probably only if user overwrites all MEDs
> on input to the AS.

Manipulating MED is not common but also not unusual. It tends to be used in
situations where manipulating LOCAL_PREF results in too many sub-optimal
paths and letting the decision fall through to ROUTER_ID would not achieve
the desired effect.

- Two different upstreams
- We want to use shortest AS_PATH where possible
- But if AS_PATH is same length we want to prefer ISP A
- We can't adjust LOCAL_PREF with a prefix filter because it's not a known set
- But ROUTER_ID would pick ISP B
- Only option is to manipulate MED and use always-compare-med

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