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Identifying VLANs

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IEEE 802.1Q

Created by the IEEE as a standard method of frame tag-

ging, it actually inserts a field into the frame to identify the VLAN. If
you're trunking between a Cisco switched link and a different brand of
switch, you have to use 802.1Q for the trunk to work.

The basic purpose of ISL and 802.1Q frame-tagging methods is to provide

interswitch VLAN communication.

LAN emulation (LANE)

This is used to communicate multiple VLANs

over ATM.

802.10 (FDDI)

Employed for sending VLAN information over FDDI, it

uses a SAID field in the frame header to identify the VLAN. This is also
proprietary to Cisco devices.

The CCNA exam covers only the ISL method of VLAN identification, but it's

good to know about the others too.

Inter-Switch Link (ISL) Protocol

Inter-Switch Link (ISL) is a way of explicitly tagging VLAN information
onto an Ethernet frame. This tagging information allows VLANs to be
multiplexed over a trunk link through an external encapsulation method,
which allows the switch to identify the VLAN membership of a frame over
the trunked link.

By running ISL, you can interconnect multiple switches and still maintain

VLAN information as traffic travels between switches on trunk links. ISL
provides a low-latency, full wire-speed performance, in contrast to Fast
Ethernet, which uses either half- or full-duplex mode.

Cisco created the ISL protocol, and it's proprietary to Cisco devices only. If you

need a non-proprietary VLAN protocol, use the 802.1Q that's covered in the

Sybex

CCNP: Switching Study Guide

www.sybex.com