VLAN Frame Tagging

VLAN Frame Tagging Protocols (ISL and dot1.q)

In this lesson, we will learn VLAN Frame Tagging Protocols detailly. In the next lessons, you can also check Cisco VLAN Configuration Examples.

What are these VLAN Frame Tagging Protocols? Let’s see each of them.

There are two types of frame tagging protocols. These are :

– ISL(Inter-Switch Link)
– dot1Q (or IEEE 802.1Q)

These protocols can be configured manually or negotiated by DTP(Dynamic Trunking Protocol). DTP is a Cisco proprietary protocol. For VLAN Tagging Cisco, DTP can be used.

ISL encapsulate the frame with a header (26 bytes) and trailer (4 bytes). So ISL increases the size of a frame 30 bytes. This protocol is a Cisco proprietary protocol and it is not supported on new Cisco devices. ISL support 1000 VLAN on a truk port.

You can configure ISL on Cisco switches like below:

Switch(config)# interface fa0/0

Switch(config-if)# switchport trunk encapsulation isl

Dot1Q (or IEEE 802.1Q) is the industry standart. So with this frame tagging protocol, you can trunk different vendors’ switches. Dot1Q modifies the layer-2 header and add 4-byte VLAN tag into it. Because of this process, the frame CRC value is calculated again. Dot1.q support 4096 VLAN s on a trunk.

You can configure dot1.q on Cisco switches like below:

Switch(config)# interface fa0/0

Switch(config-if)# switchport trunk encapsulation dot1.q

The normal frame size is 1514 bytes. With ISL, this value increate 30 bytes and the frame become giant to other vendor’s switches. But with dot1.q, the frame size become 1514 to 1518. And this value is supported by all other vendors’ switches.