Hirschmann RS20 Programming Manual page 82

Rapid Spanning Tree
When network segments are connected to a MRP ring and you enable MRP
compatibility, a peculiarity results. If the root bridge is located inside the MRP
ring, the devices inside the MRP ring are combined into one virtual device for
the purpose of calculating the branch length.
Note: The RSTP Standard dictates that all the devices within a network work
with the (Rapid) Spanning Tree Algorithm. However, if STP and RSTP are
used at the same time, the advantages of faster reconfiguration with RSTP
are lost. RSTP devices also work in a limited MSTP environment within the
scope of their functionality.
Note: Due to a change in the IEEE 802.1D-2004 standard on which RSTP is
based, the Standards Commission has reduced the maximum value for the
"Hello Time" from 10 to 2. When earlier firmware versions are upgraded to
version 5.x or higher, the firmware automatically changes a locally entered
"Hello Time" value greater than 2 to 2.
If the device is not the RSTP root, "Hello Time" values greater than 2 can re-
main valid, depending on the firmware version of the root device.
Redundanz L2P
82
Release 5.0 04/09