Allied Telesis AT-S81 - Manual (Page 140)

Chapter 12: Rapid Spanning Tree Protocol (RSTP)
140 Section II: Menus Interface
Table 4 lists the RSTP port costs with Auto-Detect.
Table 5 lists the RSTP port costs with Auto-Detect when the port is part of
a port trunk.
You can override Auto-Detect and set the port cost manually. However,
you must assign the same port cost to all ports that are members of a
trunk.
Port Priority
If two paths have the same port cost, the bridges must select a preferred
path. In some instances this can involve the use of the port priority
parameter. This parameter is used as a tie breaker when two paths have
the same cost.
The range for port priority, in hexadecimal format, is 0 to 240, with 240
being the highest priority. As with bridge priority, this range is broken into
multiples of 16. To select a port priority for a port, you enter the desired
value.
Table 4. RSTP Auto-Detect Port Costs
Port Speed Port Cost
10 Mbps 2,000,000
100 Mbps 200,000
1000 Mbps 20,000
Table 5. RSTP Auto-Detect Port Trunk Costs
Port Speed
No. of Ports/
Trunk
Port Cost
10/100 2 100,000
10/100 3 66,666
10/100 4 50,000