IEEE frame tagging standard for VLAN trunking In Ethernet switching and inter-switch trunking, which of the following is the IEEE standard that defines frame tagging for Virtual LAN (VLAN) identification?

Introduction / Context:
Modern switched networks commonly use VLANs to segment broadcast domains at Layer 2. When VLANs span multiple switches, trunk links must carry traffic for several VLANs simultaneously. To do this, Ethernet frames are tagged so that the receiving switch can identify the VLAN membership. This question asks you to identify the correct IEEE standard that specifies VLAN frame tagging.

Given Data / Assumptions:

• The context is VLAN tagging on Ethernet trunks between switches or between a switch and a router (router-on-a-stick).
• We are comparing well-known standards and terms used in switching.
• The goal is to pick the official IEEE standard for tagging.

Concept / Approach:

The IEEE 802.1Q standard defines how an additional tag is inserted into the Ethernet frame header to identify the VLAN ID and priority information. This tag allows multiple VLANs to traverse the same physical link. Other options listed refer to different technologies or non-IEEE mechanisms and thus are not the correct answer for the standard that defines tagging itself.

Step-by-Step Solution:

Verification / Alternative check:

Check any mainstream switch documentation: when configuring trunks, the encapsulation type is typically 802.1Q. Proprietary options like ISL are largely deprecated and were never standardized by IEEE.

Why Other Options Are Wrong:

• ISL: Cisco-proprietary encapsulation, not an IEEE standard.
• 802.3Z: Refers to Gigabit Ethernet over fiber (1000BASE-X), not VLAN tagging.
• 802.3U: Refers to Fast Ethernet (100BASE-TX/FX), not VLAN tagging.
• 802.1D: Spanning Tree Protocol standard, unrelated to frame tagging.

Common Pitfalls:

• Confusing ISL with 802.1Q because both tag frames; only 802.1Q is IEEE.
• Assuming any 802.3 substandard relates to VLANs; VLAN tagging is under 802.1 (bridging).

Final Answer:

802.1Q