Switch trunking mechanisms: Which listed items are the encapsulation methods actually used to carry multiple VLANs across a trunk link on Ethernet switches?

Introduction / Context:
Trunk links allow a single physical Ethernet connection to carry traffic for many VLANs. Two encapsulation methods historically provided this: IEEE 802.1Q (open standard) and ISL (Inter-Switch Link, Cisco proprietary and now largely deprecated). Understanding which technologies are true trunk encapsulations avoids misconfiguring VLAN management protocols as if they were trunks.

Given Data / Assumptions:

• The list in the stem includes “VLAN Trunking Protocol,” “VLAN,” “802.1Q,” and “ISL,” implicitly as items 1–4.
• VTP is a VLAN database management protocol, not a trunk encapsulation.
• “VLAN” itself is not an encapsulation method.

Concept / Approach:

Real trunk encapsulations: 802.1Q (tags frames with a VLAN ID) and ISL (encapsulates frames, older Cisco proprietary). VTP distributes VLAN information across switches but does not tag or encapsulate user frames; it simply rides over an existing trunk.

Step-by-Step Solution:

Verification / Alternative check:

On switches, trunk configuration shows encapsulation dot1q or isl (historically). VTP status is separate and independent from encapsulation choice.

Why Other Options Are Wrong:

Options involving 1 or 2 misclassify VTP or the concept “VLAN” as encapsulation methods.

Common Pitfalls:

Assuming enabling VTP “creates a trunk”; trunking must be configured (or negotiated) with protocols like DTP while encapsulation is 802.1Q/ISL.

Final Answer:

3 and 4