From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Sun Aug 25 2002 - 12:38:38 GMT-3
Colin is correct that MED is sent only to directly neighboring AS,
which do not pass it on (i.e., it is a nontransitive BGP attribute).
It can be set, however, differently on each border router in your AS,
or not set at all.
Having different MEDs to the same AS is a very common way of
implementing load sharing, backup, and the like: anything involving
preferred path for a particular destination.
By default, MEDs are only compared between peerings with the same AS.
If you enable it with always-compare-med, it will compare against all
directly connected AS. This feature is especially useful at
multilateral exchange points.
At 3:07 PM +0100 8/25/02, Colin Barber wrote:
>MED is sent from as AS (on your case AS1) into another AS (AS2). The MED is
>transferred between routers within AS2 (IBGP neighbors) but is not sent on
>to any EBGP neighbors. Therefore both your scenarios will compare MEDS for
>selecting the best path if the other attributes higher up in the selection
>process (weight, local preference, as_path, origin) are equal.
>
>Colin
>
>-----Original Message-----
>From: Angelo De Guzman [mailto:ghie_pogi@yahoo.com]
>Sent: 25 August 2002 13:33
>To: ccielab@groupstudy.com
>Subject: simple MED question
>
>
>Hi,
> "By default, BGP compares MED attribute of routes
>coming from neighbors in the same external AS"
>
>Is this statement true?
>
>If yes, does it do this on a per router basis or a per
>AS basis?
>
>Per router basis:
>
>R1 in AS 1 ---
> \---R3 in AS 2 (R3 compares MED)
>R2 in AS 1 ---/
>
>Per AS basis
>
>R1 in AS 1 ----R3 in AS 2
> | |
>R2 in AS 1 ----R4 in AS 2
>
>I'm not sure if R3 compares med with R4...
>
>Many Thanks,
>Angelo
>
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