Cisco Cisco IOS Software Release 12.2(14)S

feature can be used to help transparently change the autonomous system number of a BGP network and 
ensure that routes are propagated throughout the autonomous system, while the autonomous system 
number transition is incomplete. Because the local autonomous system number is not prepended to these 
routes, external routes will not be rejected by internal peers during the transition from one autonomous 
system number to another. 

BGP prepends the autonomous system number from each BGP network that a route traverses. This 
behavior is designed to maintain network reachability information and to prevent routing loops from 
occurring. Configuring this feature incorrectly could create routing loops. So, the configuration of this 
feature should only be attempted by an experienced network operator. 

You can use the BGP Hide Local-Autonomous System feature to transparently change the autonomous 
system number of a BGP network and ensure that routes can be propagated throughout the autonomous 
system while the autonomous system number transition is incomplete. 

This section contains the following procedures:

To configure a router that is running BGP with the BGP Hide Local-Autonomous System feature to not 
prepend the local autonomous system number to routes that are received from external peers, use the 
following steps. 

The no-prepend keyword should be used only to change the autonomous system number in a BGP 
network and should be deconfigured after the transition is complete because routing loops can be created 
if this feature is used incorrectly.

BGP prepends the autonomous system number from each BGP network that a route traverses. This 
behavior is designed to maintain network reachability information and to prevent routing loops from 
occurring. Configuring this feature incorrectly could create routing loops. So, the configuration of this 
feature should only be attempted by an experienced network operator.