D-Link DES-6300 User Manual - Spanning Tree Algorithm
Modular L3 Ethernet Switch User’s Guide
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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:
On Bridge Level
Root Bridge – The switch with the lowest Bridge Identifier is the Root Bridge. Naturally, you
will want the Root Bridge to be the best switch among the switches in the loop to ensure the
highest network performance and reliability.
Bridge Identifier – This is the combination of the Bridge Priority (a parameter that you can set)
and the MAC address of the switch. Example: 4 00 80 c8 00 01 00, where 4 is the Bridge
Priority. A lower Bridge Identifier results in a higher priority for the switch, and thus increases it
probably of being selected as the Root Bridge.
Designated Bridge – From each LAN segment, the attached Bridge that has the lowest Root
Path Cost to the Root Bridge is the Designated Bridge. It forwards data packets for that LAN
segment. In cases where all Switches have the same Root Path Cost, the switch with the lowest
Bridge Identifier becomes the Designated Bridge.
Root Path Cost – The Root Path Cost of a switch is the sum of the Path Cost of the Root Port
and the Root Path Costs of all the switches that the packet goes through. The Root Path Cost of
the Root Bridge is zero.
Bridge Priority – This is a parameter that users can set. The smaller the number you set, the
higher the Bridge Priority is. The higher the Bridge Priority, the better the chance the Switch will
be selected as the Root Bridge.