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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

A network administrator is setting up interswitch connectivity using standard IEEE 802.1Q trunking. Which action does IEEE 802.1Q take on Ethernet frames to identify VLAN membership as traffic traverses the trunk link?

  1. It inserts a 4-byte tag containing the VLAN ID directly into the Ethernet frame header between the Source MAC address and Type/Length fields.Answer
  2. B
    It enables Spanning Tree PortFast on the interconnect link so frames from all configured VLANs can bypass trunk encapsulation.
  3. C
    It transmits untagged frames for all VLANs and relies on Layer 2 CDP or LLDP discovery messages to identify frame VLAN membership.
  4. D
    It rewrites the source MAC address of every frame to match the configured native VLAN ID of the egress interface.

Answer

IEEE 802.1Q identifies VLAN membership by inserting a 4-byte header field into the Ethernet frame between the Source MAC address and the Length/Type fields.
IEEE 802.1Q performs internal frame tagging by inserting a 4-byte VLAN tag directly into the Ethernet header between the Source MAC address and the Length/Type fields. This tag includes the Tag Protocol Identifier (0x8100) and the 12-bit VLAN ID.

Step-by-Step Solution

1
Identify the standard frame modification mechanism specified by IEEE 802.1Q for interswitch trunking.
IEEE 802.1Q inserts an in-line 4-byte VLAN tag into the existing Ethernet header.
In-line tagging allows switches on both ends of the trunk link to recognize which VLAN the frame belongs to without wrapping the entire frame in a separate encapsulation wrapper.
2
Analyze the position and structure of the 802.1Q tag.
The tag is placed between the Source MAC Address field and the Length/Type field.
This 4-byte tag includes the Tag Protocol Identifier (TPID 0x8100) and Tag Control Information (TCI), which holds the 12-bit VLAN Identifier (VID).

Key Concept

IEEE 802.1Q Frame Tagging Mechanics
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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