From: Jonathan Hays (jhays@xxxxxxxx)
Date: Fri May 31 2002 - 15:48:28 GMT-3
Hi Group,
I have a question about MED operation. [No, I have not read all the RFCs
from start to finish. Sorry. ;-) ]
I am studying Lab #50 in Hutnik and Satterlee (2nd Edition),
specifically the MED configuration. In brief, for those without the
book, RouterA is in AS100 advertising networks 1.0.0.0 and 2.0.0.0 to
RouterB (AS200) and RouterC (AS200). Router A has direct serial links to
RouterB (192.1.1.1) and RouterC (193.1.1.1). There is also a RouterD
(AS200) with direct links to RouterB and RouterC (the topology is
diamond-shaped).
RouterA is configured with route maps so that updates for 1.0.0.0 sent
to RouterB will set MED=100 and updates for 1.0.0.0 sent to RouterC will
set MED=50 (which is preferred since it is lower).
RouterB's BGP table:
Network Next Hop Metric LocPrf Weight Path
*>i1.0.0.0 193.1.1.1 (RouterC) 50 100 0 100 i
* 192.1.1.1 (RouterB) 100 0 100 i
* i2.0.0.0 193.1.1.1 0 100 0 100 i
*> 192.1.1.1 0 0 100 i
RouterC's BGP table:
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0 193.1.1.1 50 0 100 i
*> 2.0.0.0 193.1.1.1 0 0 100 i
* i 192.1.1.1 0 100 0 100 i
Note the absence of the route to 1.0.0.0 via 192.1.1.1 in RouterC's
table. I started 'debug ip bgp updates" and then a "clear ip bgp *".
After some updates the following debug message popped up, which is
consistent with the BGP table:
02:59:20: BGP(0): 194.1.1.2 rcv UPDATE about 1.0.0.0/8 -- withdrawn
Q1. Would I be correct in assuming that this is normal behavior, that
after RouterC informs RouterB that it has a path to 1.0.0.0 with a
metric of 50, RouterB then withdraws its own advertisement to 1.0.0.0?
Q2. Although RouterB and RouterC are obviously receiving updates with
the MED set, I note that the output of "show route-map" on the
originating router (RouterA) does not indicate any matches. Why?
Thanks in advance for the advice,
Jonathan
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