Redundancy
184
7.5
Spanning Tree
RM Web
Release
6.0
07/2010
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 a 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 fails 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: When coupling network segments to an MRP-Ring and activating the
MRP compatibility, note this special condition: If the root bridge is within the
MRP-Ring, the devices within the MRP-Ring count as a single virtual device
when calculating the length of the branch.
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.
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.
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