[21666] in North American Network Operators' Group
RE: AS_PATH length and route selection (WAS: Strange BGP announcement)
daemon@ATHENA.MIT.EDU (Martin, Christian)
Tue Nov 17 22:55:03 1998
From: "Martin, Christian" <CMartin@mercury.balink.com>
To: "'Vijay Gill'" <wrath@cs.umbc.edu>,
"'I Am Not An Isp'"
<patrick@ianai.net>
Cc: "'nanog@merit.edu'" <nanog@merit.edu>
Date: Tue, 17 Nov 1998 22:29:01 -0500
> So, would anyone care to discuss (from an operational POV) what removing
> AS_PATH length from the BGP route selection algorithm will do to the
> Internet?
You determine your local policy based on the tools that you have. If
AS_PATH length were not in the algorithm by default, you could force it
in there (in Cisco IOS) using route maps and regexps. For example:
!
route-map MATCH_LONGER_THAN_5_ASPATH_HOPS permit 10
match as path 5
set local-preference 50
!
route-map DISCARD_LONGER_THAN_5ASPATH_HOPS permit
set local-preference 100
!
ip as-path access-list 5 permit ^[0-9]+\ [0-9]+\ [0-9]+\ [0-9]+\ [0-9]+_
!
This goes for any routing protocol, not just BGP.
-Chris
