Allied Telesis AT-S62 - Manual (Page 458)

Chapter 23: STP and RSTP
Section IV: Spanning Tree Protocols 458
Path Costs and Port Costs
After the root bridge has been selected, the bridges must determine if
the network contains redundant paths. If one is found, they must select a
preferred path while placing the redundant paths in a backup or
blocking state.
Where there is only one path between a bridge and the root bridge, the
bridge is referred to as the designated bridge and the port through which
the bridge is communicating with the root bridge is referred to as the
root port.
If redundant paths exist, the bridges that are a part of the paths must
determine which path will be the primary, active path, and which path(s)
will be placed in the standby, blocking mode. This is accomplished by an
determination of path costs. The path offering the lowest cost to the root
bridge becomes the primary path and all redundant paths are placed
into blocking state.
Path cost is determined through an evaluation of port costs. Every port
on a bridge participating in STP has a cost associated with it. The cost of
a port on a bridge is typically based on port speed. The faster the port,
the lower the port cost. The exception to this is the ports on the root
bridge, where all ports have a port cost of 0.
Path cost is simply the sum of the port costs between a bridge and the
root bridge.
The port cost of a port on an AT-8500 Series switch is adjustable through
the management software. For STP, the range is 0 to 65,535. For RSTP,
the range is 0 to 20,000,000.
Port cost also has an Auto-Detect 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-Detect is the default
setting. Table 11 lists the STP port costs with Auto-Detect.
Table 11 STP Auto-Detect Port Costs
Port Speed Port Cost
10 Mbps 100
100 Mbps 10
1000 Mbps 4