Only one instance of BGP can be configured on the router at a single time.
Use the neighbor ip-address shutdown commands to administratively shut down a BGP neighbor.
If you do not use the update-source option in the neighbor command, an announcement going to a neighbor uses the IP address of the exiting interface as the source address for a packet. When a router creates a packet, whether it is a routing update, a ping, or any other type of IP packet, the router does a lookup in the routing table for the destination address. The routing table lists the appropriate interface to get to the destination address. The address of this outbound interface is used as the source address of that packet by default.
if you want to use a loopback interface for an external neighbor instead of a directly connected interface, use static routes pointing at the physical address of the directly connected network (the next-hop address). In addition to the static route, you need to use the neighbor ip-address ebgp-multihop [ttl] router configuration command.