Rapid Spanning Tree
Redundancy L2E
Release
4.1
03/08
49
4 Rapid Spanning Tree
Note: The Spanning Tree protocol and the Rapid Spanning Tree protocol are
protocols for MAC bridges. They are described in the standards IEEE
802.1D-2004 and IEEE 802.1w. 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 geo-
graphical expansion and the number of network participants. Therefore, it
often makes sense to use multiple bridges, for example:
X to reduce the network load in subareas,
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
developed. 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 up to
30 seconds to reconfigure. This was no longer acceptable in time-sensitive
applications. The STP was therefore developed into the RSTP, leading to
reconfiguration times of less than a second.
Note: Standards dictate that all the bridges within a network work with the
(Rapid) Spanning Tree Algorithm. However, if STP and RSTP are used at the
same time, the advantages of faster reconfiguration with RSTP are lost.
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