First, let's look at the commands used in BGP:
router bgp <asn>
R1(config)#router bgp 120
This command enables BGP on a router and moves to the router configuration mode. The ASN can be any value between 1 and 65535. Once enabled, the BGP process must choose a router ID. By default, BGP uses the following methods priority-wise, to pick a router ID.
bgp router-idrouter subcommand