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Scaling the Switch Block

105

Switches create greater flexibility and scalability than routers can by

themselves because they define the network VLANs and VLAN port assign-
ments. You can group users into communities of interest, which are known
as VLAN organizations.

Because of switches, we don't need routers anymore, right? Wrong. In

Figure 3.3, notice that there are four VLANs, or broadcast domains. The
nodes within each VLAN can communicate with each other but not with any
other VLAN or node in another VLAN. When configured in a VLAN, the
nodes think they are actually in a collapsed backbone, as in Figure 3.2. What
do these hosts in Figure 3.2 need to do in order to communicate to a node or
host on a different network? They need to go through the router, or other
layer 3 device, just as they do when they are configured for VLAN commu-
nication, as shown in Figure 3.3. Communication between VLANs, just as in
physical networks, must go through a layer 3 device.

Scaling the Switch Block

irst introduced in Chapter 1, switch blocks represent a switch or

group of switches providing access to users. These switches then connect to
distribution layer switches, which in turn handle routing issues and VLAN
distribution.

To understand how many VLANs can be configured in a switch block,

you must understand the following factors:

Traffic patterns

Applications used

Network management

Group commonality

IP addressing scheme

Cisco recommends a one-to-one ratio between VLANs and subnets. For

example, if you have 2,000 users in a building, then you must understand
how they are broken up by subnets to create your VLANs. If you had 1,000
users in a subnet, which is ridiculous, you would create only 2 VLANs. If you
had only 100 users in a subnet, you would create around 20 VLANs or more.

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