VLAN Identification Methods

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, ISL is used for FastEthernet and Gigabit
Ethernet links only. It 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.
Identifying VLANs 459
IEEE 802.1Q Created by the IEEE as a standard method of frame tagging. 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 Fiber Distributed Data Interface
(FDDI). Uses a SAID field in the frame header to identify the VLAN. This is proprietary to
Cisco devices.
The Cisco Switching exam now covers only the ISL and 802.1Q methods of
VLAN identification, and the fact that the 2950, 3550, etc. series support only
802.1Q means that there should only be a small chance of ISL coming up on
the exam.
It is possible for a packet to move from one type of network, such as FDDI, to another, such
as Ethernet. Ethernet, FDDI, Token Ring, and ATM have standards enabling the switch to
translate one type into a different type. The configuration on the switch requires specifically
stating that VLAN 53 is the same thing as ATM ELAN 953, for example. The code for this is
derived from translational bridging.