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BGP AS number override

Latest reply: Dec 19, 2018 08:38:06 4955 3 10 0 0

Hi there, all!


This post explains the procedure for BGP AS number override. Please read the below-displayed information.


BACKGROUND


If the AS_Path attribute of a route to be advertised to a peer contains the number of the AS where the peer resides, the local router substitutes the AS number of the peer with the local AS number before advertising the route. On a BGP network, two devices have the same AS number and the same EBGP peer. After one of the two devices learns a route of the other device from the EBGP peer, the route is discarded because it carries an AS number that is the same as the local one. To address this problem, run the peer substitute-as command on the EBGP peer.


In case of a customer has several sites, it belong to the same agency, it all of them using the same AS number. If there not substitute the AS number, the CE will drop the packet which have its own AS number.


Below is an example of a customer with two sites with the same AS number (AS65101):


093640lpi5m2n27pexzmxr.png


PROCEDURE


The configuration template is as per below:


#

bgp 65001

ipv4-family { vpn-instance vpn-instance-name |      unicast }

peer { ipv4-address | group-name      } substitute-as

#


Note:


1. This command is only configured on the PE side.


2. You can send the BGP Community from the CE to PE to prevent the route back to the same site.

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ViktorG
Created Dec 7, 2018 07:31:16

Hello Guys!
Yes, we can solve the problem with AS_PATH BUT we also loose the network visibility. So, If you have the topology with the same AS# scattered over many sites I would recommend you to take one step further and tag all routes you send to PE-routers with specific community in the outbound direction and filter that communities in the inbound direction from PE. In complex scenarios, on PE, providers may combine CEs routers running BGP from the same customer in the peer groups,  and can happen that you may receive your routes back. This protection can be easily done with route-policy or xpl-route-filter (for NE40E). And the last thing to add, if you want send the community to your BGP peers with route-policy or xpl-route-filter, do not forget to send community to your eBGP peer with  peer X.X.X.X advertise-community .

Take care guys! This post was last edited by ViktorG at 2018-12-07 07:34.
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yiyi0519
Created Dec 7, 2018 08:28:53

Posted by ViktorG at 2018-12-07 07:31 Hello Guys!Yes, we can solve the problem with AS_PATH BUT we also loose the network visibility. So, ...
Thanks for your suggestion
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4am
Created Dec 19, 2018 08:38:06

This is a new knowledge for me.
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