Spanning Tree - Hirschmann RS20 User Manual

Spanning Tree

4 Spanning Tree
Note: The Spanning Tree Protocol is a protocol for MAC bridges. For this
reason, the following description employs 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 the total failure of the
network. In order to avoid this, you can use Spanning Tree. Spanning Tree
enables loop-free switching through the systematic deactivation of redundant
connections. Redundancy ensures 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 fails, 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 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.
Redundancy L2B
Release 6.0 07/2010
3.1 Sub-Ring
39