Stp / Rstp / Mstp - Siemens SIMATIC NET System Manual

Network structures and network configuration
2.3 Media redundancy
2.3.9

STP / RSTP / MSTP

Spanning Tree Protocol (STP)
STP (IEEE 802.1D standard) is the method with which loops are prevented in redundant
network structures.
With this method, it is not end devices that know the path from the sender or recipient, but
rather the switches. The switches continuously exchange configuration frames with each
other known as BPDUs (Bridge Protocol Data Unit). Through the MAC addresses of the
packets passing through, the switches learn the topology of the network by themselves. The
network is considered to be a tree.
Sequence
After initialization of the switches, a root bridge is selected. Each switch has an ID that it
passes on to the group. The switch with the lowest bridge ID becomes the root bridge.
All other paths are decided by this root bridge. The other switches select one of their ports as
a root port in the direction of the root bridge. This selection is also made using BPDUs that
the root bridge sends to the switches. The port of switch that receives the BPDU of the root
bridge first adopts the status of root port.
The designated ports are selected from the remaining ports connected to another switch.
This is also done by sending BPDUs. This time the switches send frames to the connected
partners. The port via which the frame reaches the recipient quickest becomes the
designated port.
The other port is deactivated. If there is a malfunction or device failure, the network needs to
be reconfigured. The devices start to negotiate new paths only when the interruption occurs.
This can take up to 30 seconds.
Rapid Spanning Tree Protocol (RSTP)
RSTP (IEEE 802.1D-2004 standard) is a further development of STP. RSTP 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 allows the reconfiguration time for an RSTP-controlled network to be reduced
to less than 1 second.
This was achieved by the following functions:
● Edge ports
A port that is defined as an edge port is activated immediately after connection
establishment. If a BPDU is received at an edge port, the port loses its role as edge port
and takes part in RSTP 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.
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System Manual, 09/2019, C79000-G8976-C242-10
Industrial Ethernet