Layer 2 Switching and the Spanning Tree

Layer 2 Switching and
the Spanning Tree
Protocol (STP)
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:


Understand the Physical, Data Link, and Network layer
technologies used in a multilayer switched network.


Describe Spanning Tree (STP), and explain the operation of
common and per-VLAN STP implementations.


Configure Spanning Tree in both Common Spanning Tree
(CST) and per-VLAN modes.

In this chapter, we’ll explore the three distinct functions of layer 2
switching: address filtering, forward/filter decision-making, 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.
This chapter continues the discussion of layer 2 switching started in Chapter 12, “The Campus
Network.” We will consider the different modes of switching that may be employed, move on to
see how network loops occur in a layer 2 network, and then provide an introduction to STP,
including the different components of STP and how to configure STP on layer 2 switched networks.
It is necessary for networking professionals to have a clear understanding of the STP, so
by the end of this chapter, you will know how to use STP to stop network loops, broadcast storms,
and multiple frame copies. In Chapter 16, “Using Spanning Tree with VLANs,” 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 provides consistent network
availability when a network outage occurs on one link. STP provides the necessary loopavoidance
function, but there are several additional features that can be utilized. For example,
it is possible to load-balance over the redundant links as well, and VLANs have a special part
to play, so we will continue the discussion in Chapter 16.