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3.2.2. 802.1Q Tagging
802.1Q is the standard trunking protocol that inserts a 4-byte tag into the Ethernet frame header.
Untagged Frame (Access Port):
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Tagged Frame (Trunk Port) — 4-byte tag inserted after Src MAC:
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The 4-byte 802.1Q tag (marked ► ◄) is inserted between Src MAC and EtherType:
- TPID (0x8100): Identifies this as a tagged frame
- PCP: Priority Code Point (3 bits, QoS values 0-7)
- DEI (D): Drop Eligible Indicator (1 bit)
- VLAN ID: 12 bits = 4,094 usable VLANs