Spanning Tree Protocol (STP)

Spanning Tree Protocol (STP)
Chapter 15, “Layer 2 Switching and the Spanning Tree Protocol (STP),” and Chapter 16, “Using
Spanning Tree with VLANs,” detail the STP, but some discussion is necessary here. STP is used
by layer 2 bridges to stop network loops in networks that have more than one physical link to the
same network. There is a limit to the number of links in a layer 2 switched backbone that needs
to be taken into account. As you increase the number of core switches, the problem becomes that
the number of links to distribution links must increase also, for redundancy reasons. If the core is
running the Spanning Tree Protocol, then it can compromise the high-performance connectivity
between switch blocks. The best design on the core is to have two switches without STP running.
You can do this only by having a core without links between the core switches. This is demonstrated
in Figure 12.11.
Figure 12.11 shows redundancy between the core and distribution layer without spanning
tree loops. This is accomplished by not having the two 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.