Allied Telesis AT-S39 - Manual (Page 100)

Section II: Local and Telnet Management
100
RSTP port cost also features an Auto-Detect feature. This features allows
RSTP to automatically set the port cost according to the speed of the
port, assigning a lower value for higher speeds. Auto-Detect is the
default setting on the ports when the switch is operating in RSTP. Table
6 lists the ports cost with Auto-Detect.
You can override Auto-Detect and set the port cost manually.
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 can be used as a tie-breaker when two paths
have the same cost.
In STP, the range for port priority is 0 to 255.
In RSTP, the range is 0 to 240. As with RSTP Bridge Priority, this range is
broken into increments, in this case multiples of 16. When you specify a
port priority for a port, you enter the increment of the desired value.
Table 6 RSTP Auto-Detect Port Costs
Port Speed Port Cost
10 Mbps 2 000 000
100 Mbps 200 000
1000 Mbps 20 000
Table 7 RSTP Port Priority Value Increments
Increment
Port
Priority
Increment
Port
Priority
0 0 8 128
1 16 9 144
2 32 10 160
3 48 11 176
4 64 12 192
5 80 13 208
6 96 14 224
7 112 15 240