Chapter 4: STP and RSTP
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
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 65 lists the values that are valid.
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