Which BGP attribute is used to influence inbound traffic when two peering sessions to the same AS exist?

The correct answer is determined by understanding the role of the MED (Multi-Exit Discriminator) attribute in BGP. The MED is specifically designed to influence the incoming traffic from neighboring Autonomous Systems (AS). When multiple paths to the same destination exist, the MED value is evaluated by external peers to decide which path to take. A lower MED value is preferred, which means that if two peering sessions to the same AS have different MED values set, the external peer will choose the path with the lower MED for routing traffic into their AS.

This mechanism is particularly useful in cases where an AS has multiple connections to another AS and wants to control which link should be used for incoming traffic from that neighboring AS. The configuration of the MED allows for traffic engineering efforts to optimize routing based on the desired traffic flow.

While the Weight attribute is a local parameter that affects routing decisions within the same AS and is not advertised to peers, Local Preference is utilized to influence outbound traffic decisions within the AS itself. Router ID simply identifies a BGP router and does not play a role in traffic selection. Understanding these distinctions is crucial for grasping how BGP operates in influencing routing decisions based on different attributes.