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Chapter 1

The Campus Network

core switches linked together. However, each distribution layer 3 switch has
a connection to each core switch. This means that each layer 3 switch has
two equal-cost paths to every other router in the campus network.

Scaling Layer 3 Backbones

As discussed in "Scaling Layer 2 Backbones," you'll typically find layer 2
switches connecting to layer 3 switches, which connect to the core with the
layer 2 switches. However, it is possible that some networks might have layer
2/layer 3/layer 3 designs (layer 2 connecting to layer 3 connecting to layer 3).
But this is not cheap, even if you're using someone else's money. There is
always some type of network budget, and you need to have good reason to
spend the type of money needed to build layer 3 switches into the core.

There are three reasons you would implement layer 3 switches into

the core:

Fast convergence

Automatic load balancing

Eliminate peering problems

Fast Convergence

If you have only layer 2 devices at the core layer, the STP will be used to stop
network loops if there is more than one connection between core devices.
The STP has a convergence time of over 50 seconds, and if the network is
large, this can cause an enormous amount of problems if it has just one link
failure.

STP is not implemented in the core if you have layer 3 devices. Routing

protocols, which have a much faster convergence time than STP, are used to
maintain the network.

Automatic Load Balancing

If you provide layer 3 devices in the core, the routing protocols can load bal-
ance with multiple equal-cost links. This is not possible with layer 3 devices
only at the distribution layer because you would have to selectively choose
the root for utilizing more than one path.

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