Allied Telesis AT-S20 - Manual (Page 62)

Configuring for Bridging
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3. Select Priority to get the next screen and enter 0 as the
priority number at the prompt to make Port 1 the root port.
The range is 0-255. When the designated root port is disabled or
the cable connection breaks, the STP algorithm reconfigures an
alternate path to the LAN by identifying the port with the next
lowest priority number.
4. Select Cost to get the next screen and enter a cost parameter
ranging from 1-65,535; or keep the default value.
The spanning tree algorithm uses the cost parameter in
combination with the priority to decide which bridges provide
the lowest cost path to the root bridge for that LAN.
Higher port costs are associated with ports of lower bandwidth,
and vice versa. For example, 100 is the cost for a 10 Mbps port, 10
for a 100 Mbps port, and 1 for a 1 Gbps port.
You are done with spanning tree configuration. Now that the
required parameters have been configured, bridges can make a
determination on the best single path to a destination.
A formula determines the amount of time it takes for the topology to
reconfigure, depending upon the spanning tree values you use.
Refer to the IEEE specification for details.