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n this chapter, we'll explore the three distinct functions of layer 2

switching: address filtering, forward/filter decision, and loop avoidance. We
will probe the issue of loop avoidance in depth and discuss how the Spanning
Tree Protocol (STP) works to stop network loops from occurring on your
layer 2 network.

It is very important to have a clear understanding of the Spanning Tree

Protocol. This chapter will continue the discussion of layer 2 switching
started in Chapter 1. We'll discuss how network loops occur in a layer 2 net-
work and then provide an introduction to STP, the different components of
STP, and how to configure STP on layer 2 switched networks. By the end of
this chapter, you will know how to use the STP to stop network loops,
broadcast storms, and multiple frame copies. In Chapter 5, we'll continue
discussing STP and provide the more complex and advanced configurations
used with it.

It is typical these days to create a network with redundant links. This pro-

vides consistent network availability when a network outage occurs on one
link. However, loop avoidance is needed, and STP provides this function. It
is possible to load-balance over the redundant links as well; we'll cover load-
balancing in Chapter 5.

Layer 2 LAN Switching

ou can think of layer 2 switches as bridges with more ports. Remem-

ber from Chapter 1 that layer 2 switching is hardware based, which means
it uses the Media Access Control (MAC) address from the hosts' network
interface cards (NICs) to filter the network. You should also remember how
switches use Application-Specific Integrated Circuits (ASICs) to build and
maintain filter tables.

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