802.1Q vs 802.1P (VLAN 0 Priority Tagging)
What are the differences? What is VLAN 0 priority tagging?
Can a device tagging frames with 802.1p communicate with a device tagging frames according to the 802.1q standard?
What is 802.1p or VLAN 0 Priority Tagging?
The VLAN0 Priority Tagging feature enables 802.1Q Ethernet frames to be transmitted with the VLAN ID set to zero.
These frames are called priority tagged frames. Setting the VLAN ID tag to zero allows the VLAN ID tag to be ignored and the Ethernet frame to be processed according to the priority configured in the 802.1P bits of the 802.1Q Ethernet frame header. Process and power system automation protocols that use priority tagging include IEC6180 GOOSE and SV.
What is 802.1Q Tagging?
Defines a system of VLAN tagging for Ethernet frames and also contains a provision for a quality of service (QoS) prioritization scheme known as 802.1P, which indicates the priority level of the frame.
The 802.1Q standard adds this information to the Ethernet header, as shown in the screenshot below.
The priority level values range form zero (best effort) to seven (highest).
These value can be used to prioritize different classes of traffic. The VLAN ID tag specifies the VLAN to which the frame belongs. The priority bits define the priority with which the frames are processed.
Native VLANs
When a given VLAN ID is assigned as a native VLAN on an Ethernet interface, frames in the native VLAN transmitted from the Ethernet interface are not tagged. Similarly, any untagged frames received on the Ethernet interface are associated with the VLAN ID in the 802.1Q header (above).
Since untagged frames do not contain priority bits in the Ethernet frame header, they are treated as best effort.
On ingress, Ethernet packets tagged with VLAN 0 are associated with the native VLAN of the interface.
VLAN 0 Priority Tagging and Priority Values
When VLAN 0 priority tagging is configured on the interface, the 802.1P priority bits are retained on ingress for the VLAN 0 tagged Ethernet frames.
To retain the 802.1P priority bits of the VLAN 0 Ethernet packets on egress, the egress interface must be in trunk mode, and the native VLAN should not be the same native VLAN as the ingress interface.
When these frames are received at the destination, the header is stripped off and the frame is processed as per the configuration of the 802.1P priority bits.
If the VLAN ID has a nonzero value, the header is retained and the frame is transmitted to the specified VLAN.
High priority frames are sent ahead of low priority frames.
Example Configuration
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