4.7 Spanning Tree Protocol
4.7.1 Theory
The Spanning Tree protocol can be used to detect and disable network loops, and to provide backup
links between switches, bridges or routers. This allows the switch to interact with other bridging
devices in your network to ensure that only one route exists between any two stations on the network,
and provide backup links which automatically take over when a primary link goes down. The spanning
tree algorithms supported by this switch include these versions:
STP – Spanning Tree Protocol (IEEE 802.1D)
RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w)
MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
The IEEE 802.1D Spanning Tree Protocol and IEEE 802.1w Rapid Spanning Tree Protocol allow for the
blocking of links between switches that form loops within the network. When multiple links between
switches are detected, a primary link is established. Duplicated links are blocked from use and become
standby links. The protocol allows for the duplicate links to be used in the event of a failure of the
primary link. Once the Spanning Tree Protocol is configured and enabled, primary links are established
and duplicated links are blocked automatically. The reactivation of the blocked links (at the time of a
primary link failure) is also accomplished automatically without operator intervention.
This automatic network reconfiguration provides maximum uptime to network users. However, the
concepts of the Spanning Tree Algorithm and protocol are a complicated and complex subject and must
be fully researched and understood. It is possible to cause serious degradation of the performance of
the network if the Spanning Tree is incorrectly configured. Please read the following before making any
changes from the default values.
The Switch STP performs the following functions:
Creates a single spanning tree from any combination of switching or bridging elements.
Creates multiple spanning trees – from any combination of ports contained within a single
switch, in user specified groups.
Automatically reconfigures the spanning tree to compensate for the failure, addition, or
removal of any element in the tree.
Reconfigures the spanning tree without operator intervention.
Bridge Protocol Data Units
For STP to arrive at a stable network topology, the following information is used:
The unique switch identifier
The path cost to the root associated with each switch port
The port identifier
STP communicates between switches on the network using Bridge Protocol Data Units (BPDUs). Each
BPDU contains the following information:
The unique identifier of the switch that the transmitting switch currently believes is the root
switch
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