North American Network Operators Group
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RE: AS_PATH length and route selection (WAS: Strange BGP announcement)

  • From: Martin, Christian
  • Date: Tue Nov 17 22:56:05 1998 
> 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