BGP setting

Unanswered Question
Mar 13th, 2009


I study the BGP command "neighbor local-as", it is for bgp number migration. However, I do not understand the local BGP router behaviour for sending/receiving "bgp routes" and remote bgp router sendig/receiving "bgp routes". could you give one example?


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Giuseppe Larosa Sat, 03/14/2009 - 05:43

Hello Anita,

this is an advanced topic

first of all a link to command reference

the default behavior of a BGP router is:

the router is configured with a single AS number with the router bgp ASN#

all routers that want to build a BGP session with the router need to use

neighbor remote-as ASN#.

Let's suppose ASN#= 500

Let's suppose that the company is bought by another ISP.

This ISP wants to deploy/use its AS example AS 1000.

Because a router can have only one BGP process in old times this required a change in all external BGP peers


neighbor remote-as 500


neighbor remote-as 1000

the command has been introduced to save this change on all eBGP peers.

with neighbor local-as 500 the local router can pretend to be part of AS 500 even if now it is actually in AS 1000.

the default behavior is that of modifying the AS path attribute so that AS 500 appears on learned routes

from another router in AS 1000 the AS path before migration is:

500 2000

after migration in AS 1000 the path is still

500 2000

the same by default happens on routes advertised to AS 2000.

To be noted that the remote router in AS 2000 doesn't change its behaviour it still thinks to be peering with a router in AS 500.

There are several options

neighbor {ip-address | peer-group-name} local-as [autonomous-system-number [no-prepend [replace-as [dual-as]]]]

you can have a look at the examples even if they are as often too short and specific.

Hope to help



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