Hirschmann RS20 Reference Manual page 158

Redundancy
Figure 44: Software configuration of the STAND-BY switch
Depending on the STAND-BY DIP switch position, the dialog displays
those configurations that are not possible as grayed-out. If you want to
select one of these grayed-out configurations, change the STAND-BY
DIP switch on the device to the other position.
One-Switch coupling
On the device set the 'STAND BY' dip switch to the ON position or use the
software configuration to assign the redundancy function to it.
Two-Switch coupling
Assign the device in the redundant line the DIP switch setting "STAND-
BY", or use the software configuration to assign the redundancy function
to it.
Note: For reasons of redundancy reliability, do not use Rapid Spanning
Tree and Ring/Network Coupling in combination.
158
6.2 Ring/Network Coupling
RM GUI L2E
Release 8.0 05/2013