Planet IGSW-2840 User Manual page 149

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

The path cost to the root from the transmitting port

The port identifier of the transmitting port
The switch sends BPDUs to communicate and construct the spanning-tree topology. All switches connected to the LAN on
which the packet is transmitted will receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch
uses the information in the frame to calculate a BPDU, and, if the topology changes, initiates a BPDU transmission.
The communication between switches via BPDUs results in the following:

One switch is elected as the root switch

The shortest distance to the root switch is calculated for each switch

A designated switch is selected. This is the switch closest to the root switch through which packets will be forwarded
to the root.

A port for each switch is selected. This is the port providing the best path from the switch to the root switch.

Ports included in the STP are selected.
149
User's Manual of IGSW-2840