why 802.1Q does not encapsulate the original frame?

Question

I am studying VLAN. After hours of searching, I know 802.1Q doesn't encapsulate the original frame, instead it adds a 32-bit field between the source MAC address and the“EtherType” field of the original frame. But I can't figure out why. Can somebody explain to me why 802.1Q doesn't encapsulate the original frame? Thanks a lot.

Answer 1

The predecessor to 802.1q was Cisco's ISL. ISL did fully encapsulate the frame. Which means when any device was receiving an ISL frame, it must be able to understand the ISL tag, or else the whole frame is considered malformed.

In 802.1q, the first 12 bytes of the frame, whether it is tagged or not, is always the same.

To illustrate exactly what the tag modifies, here is the Packet Capture of a frame without the tag, then the same frame with the tag:

The bracketed portion in orange is all from the original frame. The bracketed portion in green is what the 802.1q tag adds to the frame.

Notice that in both cases, the first 12 bytes are the Destination MAC address and the Source MAC address.

Moreover, in both cases, the next 2 bytes of the frame are a "EtherType" field, which indicate the next protocol encapsulated in the datagram.

This means that whether a transit device understands 802.1q tags or not, the processing for that frame does not change. Which means 802.1q tags will still "work" through a device that...

• is older, and doesn't support or understand 802.1q tags
• is not configured to read/look for a particular tag
• is built to only inspect the first 12 bytes of any frame so it can make a line-speed decision on how to forward the packet, which is the strategy in Cut-Through switching.

Overall, it allows the implementation and standardization of VLANs and VLAN Tagging without having to patch every device ever created that does Layer 2 processing to teach them how to interpret a "fully encapsulated VLAN tagging strategy" (like ISL). Instead, the devices that need to support VLANs can be patched to understand 802.1q, and all the other devices in transit can simply continue to operate without any fuss.

Granted, these days it is pretty rare to come across a host or switch that doesn't understand VLANs, but consider it from the perspective from when the concept of VLANs and Tagging were first invented.