I was working on a switch recently and noticed a physical interface configured similar to this –
ip address 10.1.1.2 255.255.255.0
This port was on a 6500 Chassis. The challenge I ran into was that I was replacing the 6500 with a switch from a different platform. Couldn’t get the switchport on the 3750 replacement switch to take the ip address command. Spent some time on the Cisco Support Community forums and found some explanations.
I found that all ports on a 650x series are layer 3 by default. All ports on a 3750 series are layer 2 by default. By doing a no switchport on the 3750 port, I was able to use the ip address command without any problems. With that command, you are changing the port on the 3750 from a layer 2 (switched) port to a layer 3 (routed) port.
I was curious was to why you would configure a physical port with an ip address. On a router, I understand, you are working with an interface that terminates on what you could think of as a point to point connection – another router or trunk connection on a switch. The explanations I received from my question on the forums explained that putting an ip address on a physical port did things such as keeping yet another spanning-tree process from running, faster convergence on a port change because of no spanning-tree convergence, no worrying about having to prune VLANs.