Ethernet frame tagging

802.1Q Distributed VLANs
802.1Q is a standard for distributing virtual LAN (VLAN) information across multiple switches. It enables you to have users in the same VLAN distributed among different switches but still being treated as if they were connected to the same switch.

To set this up, you need to add a 4-byte tag that contains the necessary VLAN information to transmitted Ethernet frames. In a typical configuration, switches are set up with trunk ports between them and these ports are configured to tag the frames as they’re transmitted. That way, tagged frames only pass between 802.1Q-compliant switches and don’t travel anywhere else on the network.

But beware of transmitting a frame that’s too large for the receiving equipment. This can occur if your switch attaches the 4-byte tag to a maximum-sized frame, thereby creating an illegally sized frame (one in excess of 1518 bytes). Most equipment will simply ignore the oversized frame. However, in some instances, older Ethernet equipment may discard any frame greater that 1518 bytes.

802.1P Data Prioritisation
With standard Ethernet, there’s no way to prioritise traffic because it’s designed to work purely on a first-come, first-served basis. 802.1P, however, is designed to add prioritisation functions to Ethernet frames, so mission-critical and delay-sensitive data can be given priority over the rest of the Ethernet network.

Here’s how data prioritisation works: The originator of the frame (i.e., server) analyses the data to be transmitted, assigns a priority class to the frame, then adds a 4-byte tag containing this priority information. Frames are then queued according to their assigned priority class. Typically, the originating device is directly connected to an 802.1P-compliant switch, which reads the information in the tag and, again, queues the frames per priority class.

802.1P has eight priority classes, with “1” being the lowest and “8,” the highest. A value of “0” means “normal service,” in other words, there’s no priority assigned. However, the number of supported priority classes varies between equipment manufacturers.

And, here’s one rule worth knowing: Switches should only perpetuate tagged frames from ports that have been configured to do so; otherwise, the tag will be removed prior to the frame’s transmittal. Like 802.1Q, 802.1P can produce illegally sized Ethernet frames.

An Ethernet frame before and after the addition of 802.1Q fields.