Configuration / Project Engineering; Technical Basics; Spanning Tree - Siemens SCALANCE W786-xPRO Operating Instructions Manual

Configuration / project engineering

5.1

Technical basics

5.1.1

Spanning Tree

Avoiding loops
The Spanning Tree algorithm detects redundant physical network structures and prevents
the formation of loops by disabling redundant paths. It evaluates the distance and
performance of a connection or bases the decisions on settings made by the user. Data is
then exchanged only over the remaining connection paths.
If the preferred data path fails, the Spanning Tree algorithm then searches for the most
efficient path possible with the remaining nodes.
Root bridge and bridge priority
The identification of the most efficient connection is always related to the root bridge, a
network component that can be considered as a root element of a tree-like network
structure. With the "Bridge Priority" parameter, you can influence the selection of the root
bridge. The computer with the lowest value set for this parameter automatically becomes the
root bridge. If two computers have the same priority value, the computer with the lower MAC
address becomes the root bridge.
Response to changes in the network topology
If nodes are added to a network or drop out of the network, this may 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. You can set the interval between two
configuration messages 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 started with the
new topology only after all the bridges have the required information.
SCALANCE W786-xPRO
Operating Instructions, Release 08/2007, C79000-G8976-C221-02
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