What is inside an 802.1Q tag?
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Four bytes inserted after the source MAC. A 2-byte TPID of 0x8100 says "tagged frame", then a 2-byte
TCI: 3 bits of priority (PCP, the CoS value 0 to 7), 1 bit DEI (drop eligible, once called CFI) and
12 bits of VLAN ID. The original EtherType follows the tag, the FCS is recalculated, and the largest
frame grows from 1518 to 1522 bytes.
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| Dest MAC 6 | Src MAC 6 | TPID 0x8100 (2) | PCP 3b | DEI 1b | VID 12b | EtherType 2 | Payload | FCS 4 |The fields in practice
- PCP / CoS. Layer 2 priority. Voice media is usually CoS 5, call signaling CoS 3. It only exists while the frame is tagged, which is why QoS designs carry the marking in DSCP at layer 3 as well.
- DEI. Marks a frame as a candidate to drop first under congestion. Rarely used in enterprise.
- VID. 0 means priority-tagged (a CoS value with no VLAN; the port's VLAN applies). 4095 is reserved.
ISL, for history
Cisco's older trunking protocol encapsulated the whole frame with a 26-byte header and a 4-byte CRC, 30 bytes in all. It is proprietary and gone from current platforms; Catalyst 9000 supports only 802.1Q.
They will ask next
What is a priority-tagged frame and where do you see one?
A frame with an 802.1Q tag whose VLAN ID is 0, used only to carry CoS. A phone configured with
switchport voice vlan dot1psends voice that way, and the switch puts it in the access VLAN.What tag does QinQ add?
An outer service tag, EtherType
0x88A8in 802.1ad, in front of the customer's0x8100tag.
Traps
- Placing the tag before the destination MAC. It sits between the source MAC and the EtherType.