The purpose of the Spanning Tree Protocol (STP) is to prevent loops on a switched or bridged network. STP is enabled on Catalyst 1900 series switches by default. In fact, one instance of Spanning Tree is enabled for each and every VLAN.
In earlier articles we examined some of the default settings relating to the Spanning […]

While our network is now loop free, STP keeps working away. BPDUs are still sent out at 2-second intervals to be sure that things are how they should be. If at any point a bridge doesn’t hear from another bridge, the topology will need to be recalculated. For example, let’s say that bridge B fails. […]

You may have noticed that we haven’t talked about loops yet. If you look back at Figure 3-5, a loop definitively exists. On each network segment, one port needs to be chosen at the Designated Port. The responsibility of the Designated Port is to act as the single interface to forward traffic destined for the […]

After the Root Bridge has been elected, it’s time for bridges to designate what are known as Root Ports. Before we can look at how Root Ports work, you need to know something about port costs. Obviously some ports are faster than others, and usually a faster port will be used to interconnect switches. As […]

The first critical concept in understanding Spanning Tree is that of the Root Bridge. In any Spanning Tree instance, there is only one Root Bridge, and it must be elected. The Root Bridge is elected in the initial exchange of BPDUs between bridging devices. But how does the Root Bridge get elected? That’s simple. In […]

Let me start by saying that there are still many network administrators out there who consider the words “Spanning Tree” to be evil. This stems from the fact that many networks were first segmented using bridges, and this is where Spanning Tree got its start. The main problem is that just like anything else; Spanning […]

A broadcast storm occurs when packets are continuously forwarded around a network, grinding it to a halt. This is most easily illustrated with a simple example. Consider the network in Figure 3-1, with two bridges (Bridge A and Bridge B) connecting network segments 1 and 2. At first glance, the network probably looks good – […]

In Chapter 2 we covered a basic overview of bridging and switching functions, including a look at how these devices help to segment a network into a number of smaller collision domains. In this chapter we’ll go many steps further, with a look at switching methods, loop avoidance, Virtual LANs, Spanning Tree Protocol, trunk connections, […]