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Local-AS Has to Be Matched by Incoming Filter-List

In a previous post I've described how you can use neighbor local-as feature to fix AS-number mismatch between adjacent autonomous systems. However, without additional options, the local-as is inserted in the AS-path of incoming BGP updates before any inbound filters. Your inbound filters thus have to match the local-as as well.

Consider, for example, the following configuration:

router bgp 65001
 neighbor remote-as 10
 neighbor local-as 20
 neighbor filter-list 1 in
ip as-path access-list 1 permit ^10$

Although the configuration looks correct, no routes are accepted from AS 10, as the inbound AS-path always contains locally prepended AS 20 as well as AS 10:

R1#show ip bgp neighbor received-routes | begin ^$
   Network     Next Hop      Metric LocPrf Weight Path
*           0             0 20 10 i

To fix this problem, you either have to include local AS in the AS-path access-list or use the no-prepend option of the neighbor local-as command.


  1. hi
    the topic on dual-as was gr8
    Would it be possible for anyone to throw some light on explaining in detail a scrnario wherein dual-as and as-overide is being used and also if possible another scenario where dual-as (using no-prepend / replace-as), as-overide and remove private-as is used ...
  2. Got the message. Will-do :)
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