FGSW-2620VM User’s Manual
4.3.2 Rapid Spaning Tree
1. Spanning Tree Protocol
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
41
