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Chapter 3

VLANs

VLAN Identification Methods

To keep track of frames traversing a switch-fabric, VLAN identification is
used to identify which frames belong to which VLAN. There are multiple
trunking methods:

Inter-Switch Link (ISL) Proprietary to Cisco switches, it is used for
FastEthernet and Gigabit Ethernet links only. Can be used on switch ports
and router interfaces as well as server interface cards to trunk a server.
Server trunking is good if you are creating functional VLANs and don't
want to break the 80/20 rule. The server that is trunked is part of all
VLANs (broadcast domains) simultaneously. The users do not have to
cross a layer 3 device to access a company shared server.

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.

LAN Emulation (LANE) Used to communicate with multiple VLANs
over ATM.

802.10 (FDDI) Used to send VLAN information over FDDI. Uses a
SAID field in the frame header to identify the VLAN. This is proprietary
to Cisco devices.

The Cisco Switching exam covers only the ISL and 802.1q methods of VLAN

identification.

Inter-Switch Link Protocol (ISL)

Inter-Switch Link Protocol (ISL) is a way of explicitly tagging VLAN infor-
mation onto an Ethernet frame. This tagging information allows VLANs to
be multiplexed over a trunk link through an external encapsulation method.
By running ISL, you can interconnect multiple switches and still maintain
VLAN information as traffic travels between switches on trunk links.

Cisco created the ISL protocol, and therefore ISL is proprietary in nature

to Cisco devices only. If you need a nonproprietary VLAN protocol, use the
802.1q, which is covered next in this chapter.

ISL is an external tagging process, which mean the original frame is not

altered but instead is encapsulated with a new 26-byte ISL header. It also

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