Which command is used to configure the EIGRP router ID?

The command used to configure the EIGRP router ID is indeed "eigrp router-id a.b.c.d." In EIGRP (Enhanced Interior Gateway Routing Protocol), the router ID is a 32-bit value typically represented in dotted-decimal format, similar to an IPv4 address. The router ID is crucial as it uniquely identifies each router within the EIGRP routing domain, allowing for proper route selection and management.

When this command is entered in router configuration mode, it explicitly defines what the router's ID will be. If not manually set, the router will usually choose its ID based on the highest IP address of the configured loopback interfaces or the highest IP address of the active interfaces if no loopback interfaces exist. Manually configuring the router ID provides consistency and can simplify troubleshooting, especially in complex networks.

The other options provided are not valid commands for setting the EIGRP router ID. "router eigrp a.b.c.d" incorrectly formats the command, as it refers to the AS (Autonomous System) number, not the router ID. The "set router-id a.b.c.d" and "configure eigrp a.b.c.d" commands do not exist in the Cisco IOS command set for EIGRP