BGP MED is not removed before advertising to external AS

Hi GS,

I am experiencing an weird scenario in our production network where the MED
is carried on to the our customer AS in an unexpected way.

(Customer) My ISP (upstream ISP)
 R1 --------- R2 --------- R3
(AS1) (AS2) (AS3)

On a peer with upstream (R3), we use an inbound route-map to set MED of 200,
so that we can influence the outgoing traffic routing policy, as we have a
number of peers with our upstream ISP. Please note that this is different
from a typical MED use, where you set MED on an outbound route-map to
influence incomming traffic from your peer.

For some reason, that MED I set is carried on to the downstream AS
(customer, R1), even though that we do not set it on the peering to R1. I
suspect that this is a bug (on R2), as I can not reproduce the same
behaviour on my lab router (different model, and different IOS).

Has anyone experienced the same problem before?

Another question, how I can be sure (what show, debug commands to use) that
MED is reset before R2 sends the update to R1, apart from going to R1 and
check the attributes for the route myself?

In the lab, on R2 the MED is set to 200 when 3.3.3.3/32 is received from R3.
On R1, I do not see that MED of 200, that means R2 must reset it to 0 before
it advertises to R1. However, in the debug ip bgp update output, I do not
see that R2 sends it with MED of 0. That's strange.

Your clarification is appreciated.

TGuy

R2#
*Mar 1 00:54:43.859: BGP(0): 23.0.0.3 rcvd UPDATE w/ attr: nexthop
23.0.0.3, origin i, metric 0, path 3
*Mar 1 00:54:43.863: BGP(0): 23.0.0.3 rcvd 3.3.3.3/32
*Mar 1 00:54:43.867: BGP(0): Revise route installing 1 of 1 routes for
3.3.3.3/32 -> 23.0.0.3(main) to main IP table

*Mar 1 00:54:43.871: BGP(0): 12.0.0.1 send UPDATE (format) 3.3.3.3/32, next
12.0.0.2, ***metric 200***, path 3
*Mar 1 00:54:43.971: BGP(0): updgrp 1 - 12.0.0.1 updates replicated for
neighbors: 23.0.0.3

R2#show ip bgp
BGP table version is 9, local router ID is 23.0.0.2
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network Next Hop Metric LocPrf Weight Path
*> 3.3.3.3/32 23.0.0.3 200 0 3 i

R1#
*Mar 1 00:54:44.227: BGP(0): 12.0.0.2 rcvd UPDATE w/ attr: nexthop
12.0.0.2, origin i, path 2 3
*Mar 1 00:54:44.231: BGP(0): 12.0.0.2 rcvd 3.3.3.3/32
*Mar 1 00:54:44.235: BGP(0): Revise route installing 1 of 1 routes for
3.3.3.3/32 -> 12.0.0.2(main) to main IP table

R1#show ip bgp
BGP table version is 8, local router ID is 12.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network Next Hop Metric LocPrf Weight Path
*> 3.3.3.3/32 12.0.0.2 0 2 3 i

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Received on Wed Mar 24 2010 - 23:40:48 ART