What are three possible causes of this problem?

The network administrator is trying to add Switch1 to the network, but the 802.1Q trunk is not coming up. Switch1 was previously tested in the laboratory and its trunk configuration worked fine. What are three possible causes of this problem? (Choose three.)

A.
The trunking configuration mode on Switch1 is set to Off.

B.
The trunking configuration mode on the other end is set to On.

C.
The trunking configuration mode on the other end is set to Desirable.

D.
Cisco Discovery Protocol is not running on the other end.

E.
There is a VTP domain name mismatch.

F.
Switch1 does not support 802.1Q.

Explanation:
There are 5 possible trunking modes for a switch port:

* Auto: this is the default mode. In this mode, a port will become a trunk port if the device the port is connected to is set to the on or desirable mode.
* Desirable: allows the port to become a trunk port if the device the port is connected to is set to the on, desirable, or auto mode
* On: sets the port to permanent trunking mode.
* Nonegotiate: sets the port to permanent trunking mode without sending Dynamic Trunking Protocol (DTP) frames
* Off: sets the port to permanent non-trunking mode

In this case, we can guess the trunking mode of Switch 1 is auto (default mode). When in the laboratory, the trunking mode of the other end is set to On or Desirable
so 2 switches can negotiate and the link becomes trunk with no problem. But when plugging to the network, other switches may have the trunking mode set to auto so
the 802.1Q trunk is not coming up -> B C are correct.

Of course these switches need to be in the same VTP domain so that they can talk with each other -> E is correct.