You are trying to configure a trunk port on an interface for 802.1Q encapsulation. However, after entering the proper command, you receive the error % Invalid input detected at '^' marker. What is wrong?

A. 802.1Q is not supported on the switch you are configuring this on.
B. The interface will not allow configuration of 802.1Q.
C. The switch only supports the ISL trunking protocol.
D. The switch only supports the 802.1Q trunking protocol.

D. This error is very common when configuring Cisco switches, since many switches only support 802.1Q and configuration is not necessary. The ISL trunking protocol is not supported on certain platforms, such as the 2900 series switches.