user manual

Chapter 4: STP and RSTP
66
Path Costs and Port Costs
After the root bridge has been selected, the bridges determine if the
network contains redundant paths and, if one is found, 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 is the primary, active path, and which path(s) are
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 other redundant paths are placed
into blocking state.
Path cost is determined by evaluating 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 the sum of the port costs between a bridge and the root
bridge.
The port cost of a port on the switch is adjustable through the AT-S111
Management software. For STP and RSTP, the range is from 0 to
200,000,000.
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 which 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 desired value. Table 2 on page 67 lists the
values that are valid.