Siemens SCALANCE X-300 Configuration Manual page 19

How media redundancy works in a ring topology
When using media redundancy, the data paths between the individual devices are
reconfigured if the ring is interrupted at one point. Following reconfiguration of the topology,
the devices can once again be reached in the resulting new topology.
In the redundancy manager, the 2 ring ports are disconnected from each other if the network
is uninterrupted. This prevents circulating data frames. In terms of data transmission, the ring
topology is a linear bus topology. The redundancy manager monitors the ring topology. It
does this by sending test frames both from ring port 1 and ring port 2. The test frames run
round the ring in both directions until they arrive at the other ring port of the redundancy
manager.
An interruption of the ring can be caused by loss of the connection between two devices or
by failure of a device in the ring.
If the test frames of the redundancy manager no longer arrive at the other ring port because
of an interruption in the ring, the redundancy manager connects its two ring ports. This
substitute path once again restores a functioning connection between all remaining devices
in the form of a linear bus topology.
As soon as the interruption is eliminated, the original transmission paths are established
again, the two ring ports of the redundancy manager are disconnected and the redundancy
clients informed of the change. The redundancy clients then use the new paths to the other
devices.
The time between the ring interruption and restoration of a functional linear topology is
known as the reconfiguration time.
If the redundancy manager fails, the ring becomes a functional linear bus.
Media redundancy methods
The following media redundancy methods are supported by SIMATIC NET products:
● HRP (High Speed Redundancy Protocol)
Reconfiguration time: 0.3 seconds
● MRP (Media Redundancy Protocol)
Reconfiguration time: 0.2 seconds
The mechanisms of these methods are similar. HRP and MRP cannot be used in the ring at
the same time.
SCALANCE X-300 / X-400
Configuration Manual, 10/2014, C79000-G8976-C187-22
Network management for industrial networks
2.3 Options of media redundancy
19