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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

When IEEE 802.1Q encapsulation is applied to an Ethernet frame traversing a trunk link, how many bytes are inserted into the original frame header for the 802.1Q tag?

  1. 4 bytesAnswer
  2. B
    2 bytes
  3. C
    8 bytes
  4. D
    12 bytes

Answer

The 802.1Q header inserts a total of 4 bytes into the original Ethernet frame header.
The correct answer is 4 bytes. An IEEE 802.1Q tag inserts 4 bytes (32 bits) into the Ethernet frame header. This includes 2 bytes for the Tag Protocol Identifier (TPID, fixed at 0x8100) and 2 bytes for Tag Control Information (TCI, containing 802.1p priority bits, DEI, and the 12-bit VLAN ID).

Step-by-Step Solution

1
Identify the structure of an IEEE 802.1Q tagged frame.
The 802.1Q header is inserted directly after the Source MAC address field.
This allows intermediate switches to identify VLAN membership without re-encapsulating the entire payload.
2
Break down the components of the 802.1Q tag.
The tag consists of a 2-byte Tag Protocol Identifier (TPID: 0x8100) and a 2-byte Tag Control Information (TCI) field containing Priority Code Point (3 bits), Drop Eligible Indicator (1 bit), and VLAN ID (12 bits).
Summing 2 bytes for TPID and 2 bytes for TCI equals a total header size of 4 bytes.

Key Concept

802.1Q Frame Tagging Header Architecture
Estimated Time:45s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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