When you need to have BGP sessions from an AS number different that the one configured with router bgp as-number. As Cisco doesn't support multiple instances of BGP like the way juniper supports it.
neighbor ip-address local-as as-number [no-prepend [replace-as [dual-as]]]
no-prepend: Does not prepend the local autonomous system number to any routes received from the eBGP neighbor.
replace-as: Prepends only the local autonomous-system number to the AS_PATH attribute. The autonomous system number from the local BGP routing process is not prepended.
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800949cd.shtml
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