Spanning Tree

Concept

Since alternative paths are desirable for backup and other purposes, IEEE and ISO have proposed a standard to solve the problem of “network loops”. The solution is called “The Spanning Tree Protocol”.

Purpose

Use Spanning Tree to change the:

Bridging Loops

Within the Spanning Tree Algorithm, switches connected in a LAN must detect potential “bridge loops”, and then remove these loops by blocking the appropriate ports to other switches. This is illustrated in the following diagram:

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An alternate path has been established by connecting Switch B in parallel with Switches A and C — this also creates a potential bridge loop. However, by using the Spanning Tree Algorithm, Switch B breaks the loop and blocks its path to segment 3.

Bridge Failure

If Switch A fails, the Spanning Tree Algorithm must be capable of activating an alternative path, such as Switch B.

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Network Extension

If the network is extended by adding Switch D, the Spanning Tree Algorithm must be capable of adapting automatically to the new topology, that is Switch B stops looping by blocking the path to segment 3.

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Port states when enabled

Each switch is identified by a switch ID, and each port (interface) on a switch is identified by a Port ID.

Ports can be either disabled or enabled. Ports that are enabled can be in one of these states:

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Disabled ports

Ports that are disabled do not adapt to solve the problem network loops using the Spanning Tree Protocol.

Spanning tree topology

The cost factor is used to calculate the distance from each port of a switch to the Root Switch. On the basis of this, each port on a switch is assigned one of these states:

Frame Propagation

By enforcing this strict hierarchy and by forwarding frames between only Root Ports and Designated Ports, the possibility of bridging loops is removed. Frames cannot be sent directly between switches (except via the Root Switch).

Seven-hop limit

In addition to the strict bridging hierarchy imposed by the Spanning Tree Algorithm, a 7-hop limit is introduced. Frames should not pass more than seven bridges and this limits the size of the bridged network.

Configuration BPDU messages

To establish the stable paths, each switch sends Configuration BPDU Frames to its neighboring switches. These Configuration BPDU messages contain information about the spanning tree topology. The contents of these frames changes only when the bridged network topology changes or has not been established.

Configuration BPDU messages propagation

When a bridged network is in a stable condition, switches continue to send Configuration BPDU Frames to its neighboring switches at regular intervals. Configuration BPDU Frames are transmitted down the spanning tree from Designated Ports to Root Ports. If a Configuration BPDU is not received by the Root Port on a switch in a predefined time interval (for example, because a switch along the path has dropped out), the port enters the Listening State to redetermine its stable path.

MAC address ageing

MAC address ageing is overruled by changes in the Spanning Tree. When the Spanning Tree is bridging and blocking, the topology of the network can change. This means the MAC addresses are changing and the Spanning Tree overrides the set MAC address ageing value.

VLANs

It is important to be aware of problems that may arise when using Spanning Tree and VLANs. The Spanning Tree can use alternative paths (such as different ports) to get messages to their destination. VLANs specify which ports can receive messages.

Warning

When using the Spanning Tree facility, use only one VLAN. Using two or more VLANs may cause unexpected alterations in your network topology.


How to

Change Spanning Tree values for the switch

Enable Spanning Tree on all ports


Procedures

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