VLAN Tagging

vlan frame tagging protocols

DTP and VLAN Frame Tagging Protocols (ISL and dot1.q)

VLAN Frame Tagging Protocols (ISL and dot1.q)

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.

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.

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.

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.

vlan frame tagging protocols, ISL and Dot1q

VLAN Frame Tagging Protocols, ISL and Dot1q

Both end must be configure with the same tagging portocol.

Lesson tags: VLANs
Back to: JNCIS > VLANs

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