Spanning Tree - Hirschmann RS20 User Manual

Spanning Tree

5 Spanning Tree
Note: The Spanning Tree Protocol is a protocol for MAC bridges. For this
reason, the following description uses the term bridge for switch.
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 loss of communication
across of 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. If a connection or a bridge becomes inoperable, the STP
required 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 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.
UM Redundancy Configuration L2E
Release 7.1 12/2011
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