802.1Q

A 32-bit tag added to a Ethernet frame to multiplex VLANs

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
┌───────────────────────────────┬─────┬─┬───────────────────────┐
│    Tag Protocol Identifier    │ PCP │D│       VLAN ID         │
└───────────────────────────────┴─────┴─┴───────────────────────┘

Terms

Tag Protocol Identifier

• 0x8100
• Same byte position as EtherType
• Tells the equipment “expect an 802.1q header”

PCP — Priority Code Point

• AKA, 802.1P priority bits
• AKA, Ethernet Class of Service
• AKA, CoS
• Normally zeros

DEI — Drop Eligible Indicator

• Bit 19
  • On or off
  • Normally off

VLAN ID

• 12 bits
• 0xFFF (4095) is reserved

VLAN ID ranges