dot1q

IEEE 802.1Q encapsulation tags Layer 2 frames for trunking
Dot1Q trunks are configured on the links between switches where we need to carry traffic for multiple VLANS
1. The default on older devices is ISL, a Cisco Proprietary trunking protocol, now obsolete
2. When the Switch forwards traffic to another Switch, it tags the layer 2 Dot1Q header with the correct VLAN
3. The receiving Switch will only forward the traffic out ports that are in that VLAN
4. The Switch removes the Dot1Q tag from the Ethernet frame when it sends it to the end host

PCP (Priority Code Point) field can be used to identify high/low priority traffic

4 bytes
First two bytes is the TPID (Tag Protocol ID), which is 0x8100
- This indicates that the frame is tagged for VLANs
Last two bytes is the TCI (Tag control information), and contains the PCP, DEI, and VID
- Priority Code Point (PCP)
  - 3-bit field identifying Class of Service (CoS)
- Drop eligible indicator (DEI)
  - 1-bit field that indicates if a frame can be dropped when the network is congested
- VLAN Identifier (VID)
  - 12-bit field identifying the frame's VLAN, allowing up to 4,094 VLANs
    - 12-bits = 4096 total possible values minus top and bottom (0 and 4095)

Metadata