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know I keep telling you this, but I've just got to be sure you never

forget it, so here I go one last time: By default, switches break up collision
domains and routers break up broadcast domains. Okay, I feel better! Now
let's move on.

In contrast to the networks of yesterday, which were based on collapsed

backbones, today's network design is characterized by a flatter architecture--
thanks to switches. So now what? How do we break up broadcast domains
in a pure switched internetwork? By creating a virtual local area network
(VLAN), that's how! A VLAN is a logical grouping of network users and
resources connected to administratively defined ports on a switch. When you
create VLANs, you are given the ability to create smaller broadcast domains
within a layer-2 switched internetwork by assigning different ports on the
switch to different subnetworks. A VLAN is treated like its own subnet or
broadcast domain, which means that frames broadcasted onto the network
are only switched between the ports logically grouped within the same
VLAN--very nice!

So, does this mean we no longer need routers? Maybe yes; maybe no. It

really depends on what you want to do. By default, all hosts in a VLAN can-
not communicate with hosts that are members of another VLAN, so if you
want inter-VLAN communication, the answer is yes--you still need a router.

In this chapter, you're going to learn exactly what a VLAN is and how

VLAN memberships are used in a switched internetwork. Also, I'm going to
tell you all about how VLAN Trunk Protocol (VTP) is used to update switch
databases with VLAN information, and how trunking is used to send infor-
mation about all VLANs across one link. And then I'll wrap things up by
discussing how you can make inter-VLAN communication happen by intro-
ducing a router into your switched internetwork.

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