It is best practice to configure the BGP ID explicitly by using protocols bgp <asn> parameters router-id <id>. If explicitly assigned, then this is the BGP ID that should be used. If it is not assigned explicitly, then the system choose one automatically. If a loopback address is configured as is not 127.0.0.1, it be used. If the loopback address is not used, then the largest IP address assigned to an interface in the system configuration be used.

So, best practice suggests that the following configuration commands be run on each BGP router to explicitly set the BGP ID. In this example, a loopback address of 10.0.0.11/32 is used.

vyatta@R1# set interfaces loopback lo address 10.0.0.11/32

vyatta@R1# set protocols bgp 100 parameters router-id 10.0.0.11

vyatta@R1# commit

vyatta@R1# show interfaces loopback
 loopback lo {
    address 10.0.0.11/32
 }

vyatta@R1# show protocols bgp
 bgp 100 {
    parameters {
        router-id 10.0.0.11
    }
 }