Hirschmann RS20 Reference Manual page 163

Redundancy 6.3 Spanning Tree
Note: RSTP reduces a layer 2 network topology with redundant paths into a
tree structure (Spanning Tree) that does not contain any more redundant
paths. One of the Switches takes over the role of the root bridge here. The
maximum number of devices permitted in an active branch (from the root
bridge to the tip of the branch) is specified by the variable Max Age for the
current root bridge. The preset value for Max Age is 20, which can be
increased up to 40.
If the device working as the root is inoperable and another device takes over
its function, the Max Age setting of the new root bridge determines the
maximum number of devices allowed in a branch.
Note: You have the option of coupling RSTP network segments to an MRP-
Ring. For this, you activate the MRP compatibility. This enables you to
operate RSTP via an MRP-Ring.
If the root bridge is within the MRP-Ring, the devices in the MRP-Ring count
as a single device when calculating the length of the branch. A device that is
connected to a random Ring bridge receives such RSTP information as if it
were directly connected to the root bridge.
Note: The RSTP standard dictates that all the devices within a network work
with the (Rapid) Spanning Tree Algorithm. If STP and RSTP are used at the
same time, the advantages of faster reconfiguration with RSTP are lost in the
network segments that are operated in combination.
A device that only supports RSTP works together with MSTP devices by not
assigning an MST region to itself, but rather the CST (Common Spanning
Tree).
Note: By changing the IEEE 802.1D-2004 standard for RSTP, the Standards
Commission reduced the maximum value for the "Hello Time" from 10 s to
2 s. When you update the Switch software from a release before 5.0 to
release 5.0 or higher, the new software release automatically reduces the
locally entered "Hello Time" values that are greater than 2 s to 2 s.
If the device is not the RSTP root, "Hello Time" values greater than 2 s can
remain valid, depending on the software release of the root device.
RM GUI L2E
163
Release 8.0 05/2013