Which command must be used to tell BGP to use a loopback interface address instead of a physical interface address as the source address for all BGP packets?

• A. neighbor ip-address update-source loopback interface-number
• B. neighbor ip-address update-source interface-number loopback
• C. update-source loopback interface-number
• D. neighbor update-source ip-address loopback

Answer: (A)

BGP uses the source IP of the TCP session to identify the neighbor, so to make all BGP packets originate from a loopback address you specify the loopback interface as the update-source for that neighbor. The correct approach is to configure the neighbor with update-source pointing to the loopback interface, for example neighbor update-source Loopback0. This forces BGP to use the loopback's IP as the source for all packets exchanged with that neighbor, which provides a stable source address even if physical interfaces flap.

This option is the right form because it directly ties a specific neighbor to a loopback as the source, whereas the other forms either place the update-source in an invalid position, omit the neighbor context, or attempt to use an incorrect syntax that doesn’t associate the loopback with a particular neighbor. When using loopbacks for eBGP, you’d also typically enable ebgp-multihop and ensure reachability to the loopbacks, but the core command to set the source to the loopback is the update-source for that neighbor.