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32
Chapter 1
The Campus Network
cores. Notice that there is no redundancy between the two core networks, so
STP will not be used on the core.
Core Size
Routing protocols are the main factor in determining the size of your core.
This is because routers, or any layer 3 device, isolate the core. Routers send
updates to other routers, and as the network grows, so do these updates, so
it takes longer to converge, or have all the routers update. Because at least
one of the routers will connect to the Internet, it's possible that there will be
more updates throughout the internetwork.
The routing protocol dictates the size of the distribution layer devices that
can communicate to the core. Table 1.2 shows a few of the more popular
routing protocols and the number of blocks each routing protocol supports.
Remember that this includes all blocks, including server, mainframe, and WAN.
Scaling Layer 2 Backbones
Typically, layer 2 switches are in the remote closets and represent the access
layer, the layer where users gain access to the internetwork. Ethernet
switched networks scale well in this environment, where the layer 2 switches
then connect into a larger, more robust layer 3 switch representing the dis-
tribution layer. The layer 3 device is then connected into a layer 2 device rep-
resenting the core. Because routing is not necessarily recommended in a
classic design model at the core, the model then looks like Table 1.3.
T A B L E 1 . 2
Blocks Supported by Routing Protocol
Routing
Protocol
Max Number
of Peers
Number of Subnet
Links to the Core
Max Number of
Supported
Blocks
OSPF
50
2
25
EIGRP
50
2
25
RIP
30
2
15
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