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(Rapid) Spanning Tree
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5 (Rapid) Spanning Tree
5.1 General function
Loops The Rapid/Spanning Tree Protocol (RSTP) is a standardized method (IEEE 802.1w/IEEE
802.1d) that enables the use of Ethernet networks with redundant data paths. Ethernet net-
works with redundant data paths form a meshed topology with initially impermissible loops.
Due to these loops, data packets can circulate endlessly within the network and can also be
duplicated. As a consequence, the network is usually overloaded due to circulating data
packets, and communication is interrupted. The meshed structure is therefore replaced by
a logical, deterministic path with a tree structure without loops using the Spanning Tree al-
gorithm. In the event of data path failure, some of the previously disconnected connections
are reconnected to ensure uninterrupted network operation.
IEEE 802.1w RSTP prevents the long timer-controlled switch-over times of STP.
Example:
In the following network topology, (six) redundant paths have been created to ensure ac-
cess to all network devices in the event of a data path failure. These redundant paths are
impermissible loops. The Spanning Tree Protocol automatically transforms this topology
into a tree by disconnecting selected ports. In this context, one of the switches is assigned
the role of the root of the tree. From this root, all other switches can be accessed via a single
data path.
Figure 5-1 Possible tree structure with Spanning Tree