Administrator Guide

Layer 3 Routing Commands 1297
Command Mode
BGP Router Configuration mode, IPv4 Address Family Configuration mode
User Guidelines
In typical data center deployments using CLOS networks, the peering is all
external BGP between the BGP devices requiring an unique ASN for each
router. Normally, the private BGP networks are expected to use private AS
numbers. But, there are only 1024 private AS numbers in the standard 2-byte
ASN.
Due to this limitation, data center deployments are forced to use public ASNs
in their private networks. When such private networks are interconnected to
each other, there needs to be a way to manipulate the public ASNs in the
route advertisements so that the private networks with the public ASNs don’t
experience ASN conflicts.
With the options
no-prepend and replace-as
• The router replaces the global AS of the router with the configured local-as
when advertising the routes to the peer on which this command is
configured.
• As well the
local-as is not prepended to the routes received from the
neighbor on which this command is configured.
This command is allowed only on external BGP neighbors. A neighbor can
inherit this configuration from a peer template.
When the local-as is configured for a peer, the BGP peer adjacency gets reset.
Command History
Introduced in version 6.3.0.1 firmware.
Example
console(config)#router bgp 65000
console(config-router)# neighbor 172.20.1.2 remote-as 65001
console(config-router)# neighbor 172.20.1.2 local-as 65002 no-prepend
replace-as
console(config-router)# neighbor 2001::2 remote-as 65003
console(config-router)# neighbor 2001::2 local-as 65002 no-prepend replace-
as
2CSNXXX_SWUM204.book Page 1297 Monday, January 25, 2016 1:25 PM