BGP neighbor local-as

The neighbor local-as command is used to customize the AS_PATH attribute by adding and removing autonomous system numbers for routes received from eBGP neighbors.
The configuration of this command allows a router to appear to external peers as a member of another autonomous system for the purpose of autonomous system number migration. This feature simplifies the process of changing the autonomous system number in a BGP network by allowing the network operator to migrate customers to new configurations during normal service windows without disrupting existing peering arrangements.

Caution BGP prepends the autonomous system number from each BGP network that a route traverses to maintain network reachability information and to prevent routing loops. This command should be configured only for autonomous system migration, and should be deconfigured after the transition has been completed. This procedure should be attempted only by an experienced network operator. Routing loops can be created through improper configuration. This command can be used for only true eBGP peering sessions. This command does not work for two peers in different subautonomous systems of a confederation.

This command supports individual peering sessions and configurations applied through peer groups and peer templates. If this command is applied to a group of peers, the individual peers cannot be customized.

Video Demonstration

from: Cisco IOS IP Routing: BGP Command Reference

neighbor local-as

To customize the AS_PATH attribute for routes received from an external Border Gateway Protocol (eBGP) neighbor, use the neighbor local-as command in address family or router configuration mode. To disable AS_PATH attribute customization, use the no form of this command.

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

no neighbor {ip-address | peer-group-name} local-as

Syntax Description

ip-address	IP address of the eBGP neighbor.
peer-group-name	Name of a BGP peer group.
autonomous-system-number	(Optional) Number of an autonomous system to prepend to the AS_PATH attribute. The range of values for this argument is any valid autonomous system number from 1 to 65535. •In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, and later releases, 4-byte autonomous system numbers are supported in the range from 65536 to 4294967295 in asplain notation and in the range from 1.0 to 65535.65535 in asdot notation. •In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and Cisco IOS XE Release 2.3, 4-byte autonomous system numbers are supported in the range from 1.0 to 65535.65535 in asdot notation only. For more details about autonomous system number formats, see the router bgp command. Note With this argument, you cannot specify the autonomous system number from the local BGP routing process or from the network of the remote peer.
no-prepend	(Optional) Does not prepend the local autonomous system number to any routes received from the eBGP neighbor.
replace-as	(Optional) Replaces the real autonomous system number with the local autonomous system number in the eBGP updates. The autonomous system number from the local BGP routing process is not prepended.
dual-as	(Optional) Configures the eBGP neighbor to establish a peering session using the real autonomous system number (from the local BGP routing process) or by using the autonomous system number configured with the autonomous-system-number argument (local-as).

Command Default

The autonomous system number from the local BGP routing process is prepended to all external routes by default.

Command Modes

Address family configuration (config-router-af)
Router configuration (config-router)

Files:
Topology
Configs
BGP Configuration