Rstp - Siemens SIMATIC NET SCALANCE SC-600 Configuration Manual

3.6.4

RSTP

Rapid Spanning Tree Protocol (RSTP)
One disadvantage of STP is that if there is a disruption or a device fails, the network
needs to reconfigure itself: The devices start to negotiate new paths only when the
interruption occurs. This can take up to 30 seconds. Fur this reason, STP was expanded
to create the "Rapid Spanning Tree Protocol" (RSTP, IEEE 802.1w). This differs from STP
essentially in that the devices are already collecting information about alternative routes
during normal operation and do not need to gather this information after a disruption has
occurred. This means that the reconfiguration time for an RSTP controlled network can
be reduced to a few seconds.
This is achieved by using the following functions:
• Edge ports (end node port)
Edge ports are ports connected to an end device.
A port that is defined as an edge port is activated immediately after connection
establishment. If a spanning tree BPDU is received at an edge port, the port loses its
role as edge port and it takes part in (R)STP again. If no further BPDU is received
after a certain time has elapsed (3 x hello time), the port returns to the edge port
status.
• Point-to-point (direct communication between two neighboring devices)
By directly linking the devices, a status change (reconfiguration of the ports) can be
made without any delays.
• Alternate port (substitute for the root port)
A substitute for the root port is configured. If the connection to the root bridge is
lost, the device can establish a connection over the alternate port without any delay
due to reconfiguration.
• Reaction to events
Rapid spanning tree reacts to events, for example an aborted connection, without
delay. There is no waiting for timers as in spanning tree.
• Counter for the maximum bridge hops
The number of bridge hops a package is allowed to make before it automatically
becomes invalid.
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.
Example for configuration
You can find an example for configuration of a mesh network based on "RSTP" on the
Internet pages of Siemens Industry Online Support.
Link: (https://support.industry.siemens.com/cs/ww/en/view/109742120)
SCALANCE SC-600 Web Based Management (WBM)
Configuration Manual, 10/2021, C79000-G8976-C475-03
Technical basics
3.6 Redundancy
49