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5.6.2 Rapid Spanning Tree
The Rapid Spanning Tree Protocol (RSTP) is an evolution of the Spanning Tree Protocol and provides for faster
spanning tree convergence after a topology change. The system also supports STP and the system will auto-detect the
connected device that is running STP or RSTP protocol.
5.6.2.1 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)
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 por tidentifier
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
User's Manual of ISW-1022M
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