Hello Anita,
this is an advanced topic
first of all a link to command reference
http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp3.html#wp1014448
the default behavior of a BGP router is:
the router is configured with a single AS number with the router bgp ASN#
all routers that want to build a BGP session with the router need to use
neighbor 10.10.10.1 remote-as ASN#.
Let's suppose ASN#= 500
Let's suppose that the company is bought by another ISP.
This ISP wants to deploy/use its AS example AS 1000.
Because a router can have only one BGP process in old times this required a change in all external BGP peers
from
neighbor 10.10.10.1 remote-as 500
to
neighbor 10.10.10.1 remote-as 1000
the command has been introduced to save this change on all eBGP peers.
with neighbor 10.10.10.2 local-as 500 the local router can pretend to be part of AS 500 even if now it is actually in AS 1000.
the default behavior is that of modifying the AS path attribute so that AS 500 appears on learned routes
from another router in AS 1000 the AS path before migration is:
500 2000
after migration in AS 1000 the path is still
500 2000
the same by default happens on routes advertised to AS 2000.
To be noted that the remote router in AS 2000 doesn't change its behaviour it still thinks to be peering with a router in AS 500.
There are several options
neighbor {ip-address | peer-group-name} local-as [autonomous-system-number [no-prepend [replace-as [dual-as]]]]
you can have a look at the examples even if they are as often too short and specific.
Hope to help
Giuseppe
