Redundancy
12.3
Spanning Tree
158
UM Config GRS
Release
8.0
09/2019
12.3
Spanning Tree
Note:
The Spanning Tree Protocol is a protocol for MAC bridges. For this reason, the following
description uses the term bridge for the device.
Local networks are getting bigger and bigger. This applies to both the geographical expansion and
the number of network participants. Therefore, it is advantageous to use multiple bridges, for
example:
to reduce the network load in sub-areas,
to set up redundant connections and
to overcome distance limitations.
However, using multiple bridges with multiple redundant connections between the subnetworks can
lead to loops and thus interruption of communication across the network. In order to help avoid this,
you can use Spanning Tree. Spanning Tree enables loop-free switching through the systematic
deactivation of redundant connections. Redundancy enables the systematic reactivation of
individual connections as needed.
RSTP is a further development of the Spanning Tree Protocol (STP) and is compatible with it. When
a connection or a bridge becomes inoperable, the STP requires a maximum of 30 seconds to
reconfigure. This is no longer acceptable in time-sensitive applications. RSTP achieves average
reconfiguration times of less than a second. When you use RSTP in a ring topology with 10 to
20 devices, you can even achieve reconfiguration times in the order of milliseconds.
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 devices 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, then the
Max age
setting of the new root bridge determines the maximum number of devices allowed in a
branch.
Note:
The RSTP standard dictates that every device within a network work with the (Rapid)
Spanning Tree Algorithm. When 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).
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