Spanning Tree - Siemens SIMATIC NET SCALANCE SC-600 Configuration Manual

Technical basics
3.6 Redundancy
3.6.3

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 IE switches / bridges. 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
Units) 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 SC-600 Web Based Management (WBM)
Configuration Manual, 10/2021, C79000-G8976-C475-03