802.1P
Layer 2 QoS.
The 3-bit PCP portion of the 802.1q tag.
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
- Same as 802.1q
0x8100- Same byte position as EtherType
- Tells the equipment “expect an 802.1q header”
PCP — Priority Code Point
- AKA, 802.1P
- AKA, Priority Bits
- AKA, Priority Tagging
- AKA, Ethernet Class of Service
- AKA, Trafic Class Expediting
- AKA, CoS
- Not zeros in this case
VLAN ID
Can be anything from 1 to 4094, but zero is reserved.
Vlan 0
- “This frame doesn’t have a vlan”
- “I wanted to use the L2 QoS features”
Seen in ISP networks.
DEI — Drop Eligible Indicator
- Bit 19
- On or off
- Still normally off