What is the effect of applying the switchport trunk encapsulation dot1q command to a port on a
Cisco Catalyst switch?

A.
By default, native VLAN packets going out this port are tagged.

B.
Without an encapsulation command, 802.1Q is the default encapsulation if DTP fails to
negotiate a trunking protocol.

C.
The interface supports the reception of tagged and untagged traffic.

D.
If the device connected to this port is not 802.1Q-enabled, it is unable to handle 802.1Q
packets.

Explanation:
Catalyst trunk port can use two types of encapsulation: 802.1q and ISL. The command switchport
trunk encapsulation do1q selects the first type. Reference:
http://www.cisco.com/en/US/tech/tk389/tk689/technologies_configuration_example09186a008009
441a.shtml
http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml