Reference Guide

If the AS number of the peer is different, the 4-byte speaker brings up the neighbor session using a reserved 2-byte ASN,23456
called AS_TRANS. The AS_TRANS is used to interop between a 2-byte and 4-byte AS number.
Where the 2-byte format is 1 to 65535, the 4-byte format is 1 to 4294967295. You can enter AS numbers using the traditional
format.
AS number migration
You can transparently change the AS number of an entire BGP network. Changing the AS number ensures that the routes
propagate throughout the network while migration is in progress. When migrating one AS to another and combining multiple AS,
an EBGP network may lose its routing to an IBGP if the AS number changes.
Migration is difficult as all IBGP and EBGP peers of the migrating network must be updated to maintain network reachability.
Local-AS allows the BGP speaker to operate as if it belongs to a virtual AS network besides its physical AS network.
Disable the local-as command after migration. Failure to disable the local-as command after migration causes the
local-as command to replace the original AS number of the system. You must reconfigure the system with a new AS number.
Router A, Router B, and Router C belong to AS 100, 200, and 300, respectively. Router A acquired Router B — Router B
has Router C as its client. When Router B is migrating to Router A, it must maintain the connection with Router C without
immediately updating Router C’s configuration. Local-AS allows Router B to appear as if it still belongs to Router B’s old
network, AS 200, to communicate with Router C.
The Local-AS does not prepend the updates with the AS number received from the EBGP peer if you use the no prepend
command. If you do not select no prepend, the default, the Local-AS adds to the first AS segment in the AS-PATH. If you use
an inbound route-map to prepend the AS-PATH to the update from the peer, the Local-AS adds first.
If Router B has an inbound route-map applied on Router C to prepend 65001 65002 to the AS-PATH, these events take place
on Router B:
● Receive and validate the update.
● Prepend local-as 200 to AS-PATH.
● Prepend 65001 65002 to AS-PATH.
Local-AS prepends before the route map to give the appearance that the update passed through a router in AS 200 before it
reaches Router B.
Configure Border Gateway Protocol
BGP is disabled by default. To enable the BGP process and start to exchange information, assign an AS number and use
commands in ROUTER-BGP mode to configure a BGP neighbor.
Layer 3
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