A. VTP
B. ISL
C. 802.1Q
D. STP
E. None of the above
Answer: D
Explanation:
Catalyst switches use the STP (spanning tree protocol) to prevent loops and to ensure data flows through a single network path. A separate instance of the STP mechanism is applied to every individual VLAN that is configured on the Catalyst switch.
Incorrect Answers:
A. VTP is the VLAN Trunking Protocol, which is used by Catalyst switches to pass VLAN information between switches. By itself, it does not provide any mechanism for detecting and preventing loops.
B, C. Inter-Switch Link (ISL) and 802.1Q are encapsulation types used for the creation of trunks. ISL is Cisco proprietary and 802.1Q is the industry standard method, but neither of these two options provide for any way to prevent a layer 2 loop in the network.