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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

During data transmission across an IEEE 802.1Q trunk link, a switch inserts a 4-byte header tag into Ethernet frames for non-native VLANs. At which specific location within the standard Ethernet II frame header is this 802.1Q tag inserted?

  1. Between the Source MAC Address and the Type/Length fieldsAnswer
  2. B
    Before the Destination MAC Address at the start of the frame
  3. C
    Directly after the Frame Check Sequence (FCS) field
  4. D
    Inside the Layer 3 IP header options field

Answer

The 802.1Q tag is inserted between the Source MAC Address and the Type/Length fields.
IEEE 802.1Q trunking inserts a 4-byte VLAN header directly into the Layer 2 Ethernet frame between the Source MAC Address field and the original EtherType/Length field. This tag includes the Tag Protocol Identifier (TPID set to 0x8100) and Tag Control Information (TCI, containing the VLAN ID and priority bits).

Step-by-Step Solution

1
Analyze standard Ethernet II frame structure
An Ethernet II frame consists of Preamble/SFD, Destination MAC, Source MAC, EtherType, Payload, and FCS.
Understanding standard frame layout is required to identify encapsulation insertion points.
2
Identify 802.1Q trunking tag placement rules
IEEE 802.1Q modifies the Layer 2 header by inserting a 4-byte VLAN tag field (TPID 0x8100 + TCI) directly following the 6-byte Source MAC address.
This location allows receiving switches to parse the Tag Protocol Identifier (TPID) in place of the original EtherType.
3
Evaluate frame check sequence recalculation
Because the Layer 2 header is modified, the sending switch recalculates the Frame Check Sequence (FCS) trailer before transmission.
Ensures the frame passes integrity checks on the receiving switch trunk port.

Key Concept

IEEE 802.1Q Frame Tagging and Header Structure
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Topics

VLAN Configuration and 802.1Q Trunking

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