Rapid Spanning Tree
Redundanz L2P
Release
5.0
04/09
81
6 Rapid Spanning Tree
Note: The Spanning Tree and Rapid Spanning Tree protocols based on
IEEE 802.1D-2004 and IEEE 802.1w respectively are protocols for MAC
bridges. For this reason, the following description of these protocols usually
employs the term bridge instead of switch.
Local networks are getting bigger and bigger. This applies to both the
geographical expansion and the number of network participants. Therefore,
it usually makes sense to use multiple bridges, for example:
X to reduce the network load in sub-areas,
X to set up redundant connections and
X 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. To prevent this, the (Rapid) Spanning Tree Algorithm was devel-
oped. The Rapid Spanning Tree Protocol (RSTP) enables redundancy by
interrupting loops.
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 requires a maxi-
mum of 30 seconds to reconfigure. This was no longer acceptable in time-
sensitive applications. The STP was therefore developed to the RSTP, lead-
ing to average reconfiguration times of less than a second. If you use RSTP
in a ring topology with 10 - 20 devices, you can achieve reconfiguration
times in the range of milliseconds.
Note: RSTP resolves a given topology to a tree structure (Spanning Tree).
The number of devices in a branch (from the root to the branch tip) is limited
by the parameter Max Age. The default value for Max Age is 20, it can be
increased to 40.
You should note the following here: If the root device fails and another device
takes over the root function, the largest possible number of devices decreas-
es accordingly.
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