From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Wed Sep 10 2003 - 18:10:38 GMT-3
Mike,
Multi Exit Discriminator (MED) is an optional, non-transitive
attribute. This means that the MED value is not maintained when
advertising a prefix to an EBGP neighbor. Think of MED as being in the
community no-export. Since in your case R5 is an EBGP neighbor for both
R2 and R4, the MED value is not passed when R2 and R4 are advertising
prefixes learned from R1 and R3.
The issue that R1 and R3 are in different AS's is separate. By
default, MED is only compared from prefixes learned from the same AS.
If the 'bgp always-compare-med' command is issued, MED is always
compared regardless of which AS the prefix is learned from. If the 'bgp
deterministic-med' command is applied MED is first in the decision
process among routes learned from the same AS.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 708-362-1418 (Outside the US and Canada)
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Nordhoff, Michael G. (US - Hermitage)
Sent: Wednesday, September 10, 2003 3:39 PM
To: 'ccielab@groupstudy.com'
Subject: BGP Question Concerning MED attribute
A quick question concerning the BGP MED attribute...
Is it normal behavior for a BGP peer to NOT pass along MED information
for
prefixes learned from BGP peers in different AS's? My scenario is
this...
R1 is attached to R2. R3 is attached to R4. R2 and R4 are attached to
R5.
R1 advertises a prefix to R2 via EBGP as does R3 to R4. R2 and R4 then
advertise these prefixes to R5 via EBGP. The MED attributes added by R1
and
R3 are received by R2 and R4, respectively. However, these MEDs are not
being passed along to R5. What is the reason for this behavior and what
can
be done to circumvent it?
Thanks in advance!
Mike Nordhoff - CCNP,CCDP
Deloitte & Touche, National WAN Services
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