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Chapter 20: Spanning Tree and Rapid Spanning Tree Protocols
218 Section V: Spanning Tree Protocols
Table 16 lists the STP port costs with Auto-Detect when a port is part of a
port trunk.
Table 17 lists the RSTP port costs with Auto-Detect.
Table 18 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.
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 is 0 to 240. As with bridge priority, this range is
broken into increments, in this case multiples of 16. To select a port
priority for a port, you enter the increment of the desired value. Table 19
lists the values and increments. The default value is 128, which is
increment 8.
Table 16. STP Auto-Detect Port Trunk Costs
Port Speed Port Cost
10 Mbps 4
100 Mbps 4
1000 Mbps 2
Table 17. RSTP Auto-Detect Port Costs
Port Speed Port Cost
10 Mbps 2,000,000
100 Mbps 200,000
1000 Mbps 20,000
Table 18. RSTP Auto-Detect Port Trunk Costs
Port Speed Port Cost
10 Mbps 20,000
100 Mbps 20,000
1000 Mbps 2,000