Siemens SCALANCE W786-xPRO Operating Instructions Manual page 140

Configuration / project engineering
5.6 Configuration with Web Based Management
Definition of the network topology using the configuration frames
Network components 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. The root bridge is the bridge that controls the
spanning tree algorithm for all involved components. BPDUs also bring about the status
change of the bridge ports.
Rapid Spanning Tree
The rapid spanning tree algorithm is based on the spanning tree algorithm. This was
optimized in terms of the reconfiguration time. Typical reconfiguration times for Spanning
Tree are between 20 and 30 seconds. With rapid spanning tree, the reconfiguration times
are around 1 second. This was achieved by the following measures:
● Edge ports
● Point to Point (direct communication between two neighboring stations)
● Alternate port (substitute for the root port)
● Filter table
● Reaction to events
In principle, therefore with rapid spanning tree, alternatives for many parameters are
preconfigured and certain properties of the network structure taken into account to reduce
the reconfiguration time.
(Rapid) Spanning Tree configuration
The parameters used for the (Rapid) Spanning Tree protocol are displayed in the "(Rapid)
Spanning Tree Properties" menu.
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A port defined as an edge port is activated after the hello time (the time between two
configuration frames). When the hello time has elapsed, the station can be certain that no
further configuration frame will arrive and that this port is an edge port. If the user wants
to avoid the hello time, spanning tree can be disabled at this port.
By directly linking network components, a status change (reconfiguration of the ports) can
be made without any delays. A point-to-point connection can, for example, be a WDS
connection between two access points.
A substitute for the root port is configured. If the connection to the root bridge is lost, the
station can establish a connection over the alternate port without any delay by
reconfiguring.
In rapid spanning tree, ports affected by a reconfiguration are immediately deleted from
the filter table. With spanning tree, on the other hand, the point at which a port is deleted
is decided by the time when the port was entered in the filter table.
Rapid spanning tree reacts to events, for example an aborted connection, without delay.
There is no waiting for timers as in spanning tree.
Operating Instructions, Release 08/2007, C79000-G8976-C221-02
SCALANCE W786-xPRO