Chapter 36: Spanning Tree and Rapid Spanning Tree Protocols
470 Section VI: Spanning Tree Protocols
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.
A bridge that has only one path between itself and the root bridge is
referred to as the designated bridge. And the port through which it 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 the redundant paths are placed in
the blocking state.
Path cost is determined by evaluating port costs. Every port on a bridge
participating in STP and RSTP 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 is adjustable on the switch. The range is 6 to 40.
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