What is the first priority for selecting the EIGRP Router ID?

The first priority for selecting the EIGRP Router ID is manual configuration. When you manually configure a router ID, it is explicitly set and takes precedence over any automatically assigned values. This allows network administrators to define a specific Router ID, which can help maintain consistency in network design or when troubleshooting.

If a Router ID isn't manually configured, EIGRP then assesses other criteria in the following order: it will look for the highest IP address configured on any of the router's loopback interfaces. If there are no loopback interfaces, it will fall back to the highest IP address found on any physical interface. This automated selection process ensures that routers have a unique identifier in the EIGRP routing domain.

Relying on automatic selections like physical or loopback IP addresses can lead to unforeseen changes if network interfaces are modified or removed, which is why manual configuration offers more control over your EIGRP setup.