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| 3.1 |  | Spanning-Tree Protocol Operation |
| 3.1.5 |  | Path Cost |

The Path Cost is the second parameter used by the spanning-tree algorithm to determine the path to the Root Bridge. The IEEE 802.1D standard originally defined the cost of a link as 1000 Mbps divided by the bandwidth of the link in Mbps. For example, a 10BASE-T link has a cost of 100 (1000/10), and Fast Ethernet and FDDI each have a cost of 10 (1000/100). With the advent of Gigabit Ethernet and other high-speed technologies, a problem with this definition evolved.

The cost is stored as an integer value, not as a floating-point value. For example, 10 Gbps results in 1000 Mbps ÷10000 Mbps = .1, an invalid cost value. To solve this problem, the IEEE changed the “inverse proportion” definition. Figure shows the definition of the STP cost for a link exceeding 1 Gbps. Figure shows the definition of the STP cost for a link of 10 Gbps or higher.

Path Cost is a measure of how close bridges are to each other. Path Cost is the sum of the costs of the links in a path between two bridges. It is not a measure of hop count. The hop count for path A might be greater than the hop count for path B, while the cost of path A is less than the cost of path B. Closeness is not necessarily reflected by hop count.

The Path Cost is used by switches to determine the best path to the Root Bridge. The lowest combination of links will have the lowest cumulative path cost and will be the best path to the Root Bridge.