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Chapter 6

Virtual LANs (VLANs)

Frame Tagging

You can also create your VLANs to span more than one connected switch.
In Figure 6.4, hosts from various VLANs are spread across many switches.
This flexible, power-packed capability is probably the main advantage to
implementing VLANs!

But this can get kind of complicated--even for a switch--so there needs

to be a way for each one to keep track of all the users and frames as they
travel the switch fabric and VLANs. (Remember, a switch fabric is a group
of switches sharing the same VLAN information.) This is where

frame

tagging

comes in. This frame identification method uniquely assigns a user-

defined ID to each frame. Sometimes people refer to it as a "VLAN ID"
or "color."

Here's how it works: Each switch that the frame reaches must first iden-

tify the VLAN ID from the frame tag, then it finds out what to do with the
frame by looking at the information in the filter table. If the frame reaches
a switch that has another trunked link, the frame will be forwarded out the
trunk-link port.

Once the frame reaches an exit to an access link, the switch removes the

VLAN identifier. This is so the destination device can receive the frames
without having to understand their VLAN identification.

VLAN Identification Methods

So VLAN identification is what switches use to keep track of all those frames
as they're traversing a switch fabric. It's how switches identify which frames
belong to which VLANs, and there's more than one trunking method:

Inter-Switch Link (ISL)

This is proprietary to Cisco switches, and it's

used for Fast Ethernet and Gigabit Ethernet links only.

ISL routing

can

be used on a switch port, router interfaces, and server interface cards to
trunk a server. This is a very good approach if you're creating functional
VLANs and you don't want to break the 80/20 rule.

Wait a minute--what's the 80/20 rule? Well, it's a formula that says 80 per-
cent of the data traffic should stay on the local segment while 20 percent
or less crosses a segmentation device. A trunked server is part of all VLANs
(broadcast domains) simultaneously, so users don't have to cross a layer-3
device to access it.

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