Redundancy Mechanism; Spanning Tree - Siemens SCALANCE XB-200 Web Based Management

Technical basics

4.4 Redundancy mechanism

4.4 Redundancy mechanism
4.4.1

Spanning Tree

Avoiding loops on redundant connections
The spanning tree algorithm allows network structures to be created in which there are
several connections between two stations. Spanning tree prevents loops being formed in the
network by allowing only one path and disabling the other (redundant) ports for data traffic. If
there is an interruption, the data can be sent over an alternative path. The functionality of the
spanning tree algorithm is based on the exchange of configuration and topology change
frames.
Definition of the network topology using the configuration frames
The devices exchange configuration frames known as BPDUs (Bridge Protocol Data Unit)
with each other to calculate the topology. The root bridge is selected and the network
topology created using these frames. BPDUs also bring about the status change of the root
ports.
The root bridge is the bridge that controls the spanning tree algorithm for all involved
components.
Once the root bridge has been specified, each device sets a root port. The root port is the
port with the lowest path costs to the root bridge.
Response to changes in the network topology
If nodes are added to a network or drop out of the network, this can affect the optimum path
selection for data packets. To be able to respond to such changes, the root bridge sends
configuration messages at regular intervals. The interval between two configuration
messages can be set with the "Hello Time" parameter.
Keeping configuration information up to date
With the "Max Age" parameter, you set the maximum age of configuration information. If a
bridge has information that is older than the time set in Max Age, it discards the message
and initiates recalculation of the paths.
New configuration data is not used immediately by a bridge but only after the period
specified in the "Forward Delay" parameter. This ensures that operation is only started with
the new topology after all the bridges have the required information.
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SCALANCE XB-200 Web Based Management
Configuration Manual, 04/2015, C79000-G8976-C360-02