Rstp, Mstp, Cist - Siemens SCALANCE XM-400 Configuration Manual

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.
4.6.1.1

RSTP, MSTP, CIST

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.
SCALANCE XM-400/XR-500 Web Based Management (WBM)
Configuration Manual, 05/2017, C79000-G8976-C248-12
Technical basics
4.6 Redundancy mechanism
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