Allied Telesis AT-S87 - Manual (Page 96)

Chapter 10: STP and RSTP
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port on a bridge is typically based on port speed. The faster the port, the
lower the port cost. The exceptions to this are:
ˆ The ports on the root bridge, where all ports have a port cost of 0.
ˆ When a port is a member of a trunk, the port cost of each trunk
member is the auto port cost divided by the number of trunk members.
Path cost is simply the sum of the port costs between a bridge and the root
bridge.
Port cost also has an Auto feature. This feature allows spanning tree to
automatically set the port cost according to the speed of the port,
assigning a lower value for higher speeds. Auto is the default setting.
Table 6 lists the STP port costs with Auto. When a port is an active
member of a trunk, the port cost is equal to the auto port cost divided by
the number of ports in the trunk.
Table 6 lists the RSTP port costs with Auto.
Table 7 lists the RSTP port costs with Auto when the port is part of a port
trunk.
You can override Auto and set the port cost manually.
Table 5. STP Auto Port Costs
Port Speed Port Cost
10 Mbps 2,000,000
100 Mbps 200,000
1000 Mbps 20,000
Table 6. RSTP Auto Port Costs
Port Speed Port Cost
10 Mbps 2,000,000
100 Mbps 200,000
1000 Mbps 20,000
Table 7. RSTP Auto Port Trunk Costs
Port Speed Port Cost
10 Mbps 20,000
100 Mbps 20,000
1000 Mbps 2,000