From: Simon Baxter (Simon.Baxter@xxxxxxxxxxxxxx)
Date: Tue Jul 25 2000 - 00:17:24 GMT-3
I may be a sucker for punishment, but appear to have found an error on CCO
with regard to path selection and MED attribute.
The following link describes a scenario where 'bgp always-compare-med' can
be used to 'vote' path selection based on 3 sources of the route from 2
external ASs. One of the ASs is sending updates via t points and the other
by only one.
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#xtocid27651
26
the doc states :
Unless otherwise specified, the router compares MED attributes for paths
from external neighbors that are in the same AS. If you want MED attributes
from neighbors in other ASs to be compared, you must configure the bgp
always-compare-med command.
and then :
Router A will choose Router C as the best path for reaching network
180.10.0.0. To force Router A to include updates for network 180.10.0.0 from
Router B in the comparison, use the bgp always-compare-med router
configuration command, as in the following modified configuration for Router
A:
But this is crap because the BGP path selection mechanism states that
AS-path length is more important than MED - so the only way to get an
'indirect' path MED considered is to AS-path prepend the updates from the
'direct' path so the as-path length are the same.
They should proof their docs!!!
am I right? (self doubt - no 'god' complex here!!!)
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