10/100 Fast Ethernet Switch User’s Guide
28 Switch Management
Spanning Tree Algorithm
The Spanning Tree Algorithm (STA) in the Switch allows you to
create alternative paths (with multiple switches or other types of
bridges) in your network. These backup paths are idle until the
Switch determines that a problem has developed in the primary
paths. When a primary path is lost, the Switch providing the
alternative path will automatically go into service with no
operator intervention. This automatic network reconfiguration
provides maximum uptime to network users. The concept of the
Spanning Tree Algorithm is a complicated and complex subject
and must be fully researched and understood. Please read the
following before making any changes .
♦ Network loop detection and prevention.
With STA, there will be only one path between any two
LANs. If there is more than one path, forwarded packets
will loop indefinitely. STA detects any looped path and
selects the path with the lowest path cost as the active path,
while blocking the other path and using it as the backup
path.
♦ Automatic topology re-configuration.
When the path for which there is a backup path fails, the
backup path will be automatically activated, and STA will
automatically re-configure the network topology.
STA Operation Levels
STA operates on two levels: the bridge level and the port level. On
the bridge level, STA calculates the Bridge Identifier for each
Switch, then sets the Root Bridge and the Designated Bridges. On
the port level, STA sets the Root Port and Designated Ports.
Details are as follows:
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