Which command starts configuration mode for BGP?

The command that starts configuration mode for Border Gateway Protocol (BGP) is "router bgp ASN." This command is essential because it initiates the BGP process on a router and configures it to operate in BGP as part of a specific autonomous system (AS). In this command, "ASN" refers to the Autonomous System Number, which uniquely identifies the BGP routing instance within a specific AS.

Once this command is executed, the router enters the BGP configuration mode, allowing the network administrator to set up BGP neighbors, define routing policies, and manage various configuration settings related to BGP operation. This is a foundational step in establishing BGP peering relationships and enabling the exchange of routing information between the BGP-enabled routers.

The other options do not initiate BGP configuration. For instance, "neighbor neighbor IP" is used to configure a specific BGP peer after already being in BGP configuration mode. "config t" is a general command that enters global configuration mode but does not specifically target BGP. Lastly, "exit" is used to leave a configuration mode rather than entering one. Hence, the command "router bgp ASN" is the definitive command to start BGP configuration on a router.