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Chapter 1
The Campus Network
Switch Block
The switch block is a combination of layer 2 switches and layer 3 routers. The
layer 2 switches connect users in the wiring closet into the access layer and
provide 10 or 100Mbps dedicated connections; 1900/2820 and 2900 Cata-
lyst switches can be used in the switch block.
From here, the access layer switches will connect into one or more distri-
bution layer switches, which will be the central connection point for all
switches coming from the wiring closets. The distribution layer device is
either a switch with an external router or a multi-layer switch. The distribu-
tion layer switch will then provide layer 3 routing functions, if needed.
The distribution layer router will prevent broadcast storms that could
happen on an access layer switch from propagating throughout the entire
internetwork. The broadcast storm would be isolated to only the access layer
switch in which the problem exists.
Switch Block Size
To understand how large a switch block can be, you must understand the
traffic types and the size and number of workgroups that will be using them.
The number of switches that can collapse from the access layer to the distri-
bution layer depend on the following:
Traffic patterns
Routers at the distribution layer
Number of users connected to the access layer switches
Distance VLANs must traverse the network
Spanning tree domain size
If routers at the distribution layer become the bottleneck in the network
(which means the CPU processing is too intensive), the switch block has
grown too large. Also, if too many broadcasts or multicast traffic slow down
the switches and routers, your switch blocks have grown too large.
A large number of users does not determine whether the switch block is too
large, the amount of traffic going across the network does.
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