802.1Q or dot1q Trunking Protocol Explained

IEEE 802.1Q is a trunking protocol. It is also known as dot1Q. It provides encapsulation on trunk ports. Trunk ports work with multiple VLANs. They use the trunking protocol to manage VLAN information on Ethernet frames.

All modern Cisco routers and switches use 802.1Q as the default trunking protocol to encapsulate and de-encapsulate VLAN information on Ethernet frames. By default, a switch port works only with a single VLAN. A trunking protocol allows it to work with multiple VLANs.

Key points:-

• It is an IEEE protocol.
• It works on all routers and switches.
• It works with all types of Ethernet links.
• It inserts a 4-byte tag field into the frame's header.
• It functions at the data-link layer of the OSI model.
• It is the default encapsulation protocol for VLANs on all Cisco devices.

How the 802.1Q trunking protocol works

802.1Q trunking protocol modifies the original Ethernet frame. It inserts a 4-byte field called a tag field into the header of the original Ethernet frame and recomputes the original frame's FCS (checksum) based on this change.

The following outline explains the purpose of all bits in this tag.

• The first 2 bytes are the protocol identifier. For example, an Ethernet frame will use a value 0x8100 to indicate that this is an Ethernet-tagged frame.
• The following 3 bits prioritize the frame. The IEEE 802.1p standard defines it.
• The fourth bit is for the Token Ring environment.
• The last 12 bits are the VLAN identifier (number).

On the sending device, it inserts a tag into the frames' header. The receiving device uses the VLAN ID stored in the tag to identify the VLAN to which the frame belongs. It removes the tag from frames before forwarding them from the VLAN ports they belong to.

Let us take an example.

The following image shows a network. This network has two Ethernet switches with VLAN configuration. Both have a connection through a trunk link. Trunk ports on both switches are using the 802.1Q trunking protocol. Both have end devices from VLAN-10.

PC-A sends a broadcast message. The broadcast message reaches the S1 switch's F0/1 port. The S1 switch checks the configured VLAN on the F0/1 port. The F0/1 port belongs to VLAN-10. S1 forwards the broadcast message from all ports that belong to VLAN-10. A trunk port belongs to all VLANs. It forwards frames of all VLANs to the device connected on the other end of the trunk link. It inserts a tag into all frames before forwarding them. S2 receives encapsulated frames from S1 and reads the tag field of each frame to know its VLAN. After reading VLAN information from each frame, it removes the tag from the frame and forwards it from the ports that belong to the frame's VLAN. For example, if the frame belongs to VLAN-10, it forwards the frame from ports that belong to VLAN-10.

Advantages of 802.1Q

Since it adds only 4 bytes to the frame, the frame size does not exceed 1518 bytes.The standard Ethernet frame size is 1518 bytes. All ports can send and receive standard-sized frames. Frames larger than the standard size need Fast Ethernet or higher ports.

It is an IEEE standard. It allows you to create trunks between different vendors' devices. Therefore, you can implement a multi vendor trunking solution without having to worry about whether a specific type of trunk connection is supported.

Conclusion

This tutorial explained 802.1Q. 802.1Q is a trunking encapsulation protocol. It is an open standard protocol. It works on all vendor switches.