Which BGP configuration option is required when peering with an IBGP neighbor using loopback addresses?

When configuring BGP peering with an IBGP neighbor using loopback addresses, the 'local-address' option is essential. This option allows you to specify the IP address that the local BGP speaker will use to establish the peering session.

In the case of loopback addresses, which are often used for their stability (as they are not tied to a specific physical interface), the 'local-address' option ensures that the BGP session can correctly identify which loopback IP will be utilized for the peering connection. By default, BGP would attempt to use the IP address associated with the interface on which the BGP neighbor command was issued. However, loopback addresses provide a more resilient and persistent means of establishing BGP sessions across routers in a network.

This is particularly important in IBGP configurations, where maintaining the peering session reliably is key for exchanging routing information between routers in the same Autonomous System (AS).

Other options, while they may appear relevant, do not fulfill this specific requirement of designating the use of loopback interfaces for BGP peering. Therefore, setting the 'local-address' appropriately is crucial for ensuring successful BGP neighbor relationships when using loopback addresses.