local_preference for transit traffic?

Holmes,David A dholmes at mwdh2o.com
Thu Dec 15 06:31:29 UTC 2011

For this very reason I have advocated using longest prefix BGP routing for some years now, and checking periodically for the expected path, as it became obvious from investigating traceroutes that traffic was not being routed as intended using AS prepends.

-----Original Message-----
From: Keegan Holley [mailto:keegan.holley at sungard.com]
Sent: Wednesday, December 14, 2011 10:08 PM
Subject: local_preference for transit traffic?

Had in interesting conversation with a transit AS on behalf of a customer
where I found out they are using communities to raise the local preference
of routes that do not originate locally by default before sending to a
other larger transit AS's.  Obviously this isn't something that was asked
of them and it took a few days to find since the customer is not a large
company and neither them nor my company has a link or business relationship
with the AS in question.  This seemed strange to me for obvious reasons,
but I was curious if anyone else was doing this and why.  You obviously
cannot use prepend to affect transit traffic again for obvious reasons.
MED is a weak metric but it at least only affects traffic that was already
going to transit your AS.  The larger transit AS was favoring a lower
bandwidth link for the customer and causing them to drop packets
mysteriously.  Just wondering if this practice seemed as strange to others
as it does to me.

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