для Cisco Cisco IOS Software Release 12.2(14)S

This document assumes that BGP is enabled and peering has been established in all participating 
networks.

This feature can be configured for only external BGP (eBGP) peers. 

This feature should be deconfigured after the transition to the new autonomous system number is 
completed to minimize the possible creation of routing loops. 

To configure the BGP Hide Local-Autonomous System feature, you must understand the following 
concepts:

Changing the autonomous system number may be necessary when 2 separate BGP networks are 
combined under a single autonomous system. This typically occurs when one ISP purchases another ISP. 
The neighbor local-as command is used initially to configure BGP peers to support 2 local autonomous 
system numbers to maintain peering between 2 separate BGP networks. This configuration allows the 
ISP to immediately make the transition without any impact on existing customer configurations. 

When the customer configurations have been updated, The next step is to complete the transition from 
the old autonomous system number to the new autonomous system number. However, when the 
neighbor local-as command is configured on a BGP peer, the local autonomous system number is 
automatically prepended to all routes that are learned from eBGP peers by default. This behavior, 
however, makes changing the autonomous system number for a service provider or large BGP network 
difficult because routes, with the prepended autonomous system number, will be rejected by internal 
BGP (iBGP) peers that are configured with the same autonomous system number. For example, if you 
configure an iBGP peer with the neighbor 10.0.0.2 local-as 20 statement, all routes that are learned from 
the 10.0.0.2 external peer will automatically have the autonomous system number 20 prepended. Internal 
routers that are configured with the autonomous number 20 will detect these routes as routing loops and 
reject them. This behavior requires you to change the autonomous system number for all iBGP peers at 
the same time. 

The BGP Hide Local-Autonomous System feature introduces the no-prepend keyword to the neighbor 
local-as command. The use of the no-prepend keyword will allow you to configure a BGP speaker to 
not prepend the local autonomous system number to any routes that are received from eBGP peers. This