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Spanning-Tree Protocol / Shared media environments

Spanning-Tree Protocol
8.1.6 This page will introduce STP.


When multiple switches are arranged in a simple hierarchical tree, switching loops are unlikely to occur. However, switched networks are often designed with redundant paths to provide for reliability and fault tolerance. Redundant paths are desirable but they can have undesirable side effects such as switching loops. Switching loops are one such side effect. Switching loops can occur by design or by accident, and they can lead to broadcast storms that will rapidly overwhelm a network. STP is a standards-based routing protocol that is used to avoid routing loops. Each switch in a LAN that uses STP sends messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence. This information is used to elect a root bridge for the network. The switches use the spanning-tree algorithm (STA) to resolve and shut down the redundant paths.

Each port on a switch that uses STP exists in one of the following five states:

• Blocking
• Listening
• Learning
• Forwarding
• Disabled

A port moves through these five states as follows:

• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
• From forwarding to disabled

STP is used to create a logical hierarchical tree with no loops. However, the alternate paths are still available if necessary.

This page concludes this lesson. The next lesson will discuss collision and broadcast domains. The first page covers shared media environments.

Shared media environments
8.2.1 This page explains Layer 1 media and topologies to help students understand collisions and collision domains.


Here are some examples of shared media and directly connected networks:

• Shared media environment – This occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire or optical fiber, they all share the same media environment.

• Extended shared media environment – This is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple access or longer cable distances.

• Point-to-point network environment – This is widely used in dialup network connections and is most common for home users. It is a shared network environment in which one device is connected to only one other device. An example is a PC that is connected to an Internet service provider through a modem and a phone line.

Collisions only occur in a shared environment. A highway system is an example of a shared environment in which collisions can occur because multiple vehicles use the same roads. As more vehicles enter the system, collisions become more likely. A shared data network is much like a highway. Rules exist to determine who has access to the network medium. However, sometimes the rules cannot handle the traffic load and collisions occur.

The next page will focus on collision domains.

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