5.1 Basic STP Operations
5.1.5 Path cost
Bridges use the concept of cost to evaluate how close they are to other bridges. The 802.1D standard originally defined cost 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 Fiber Distributed Data Interface (FDDI) have a cost of 10 (1000/100). This scheme has served the world well since Radia Perlman first began working on STP in 1983. However, with the implementation of Gigabit Ethernet and OC-48 ATM (2.4 Gbps), a problem has surfaced: the cost is stored as an integer value that does not account for fractional cost values. For example, OC-48 ATM results in 1000 Mbps/2400 Mbps =0.41667, 0.41667 is an invalid cost value. Which means it would be rounded up to 1, along with every technology that has a bandwidth of 1 Gbps and over. One option is to use a cost of 1 for all links equal to or greater than 1 Gbps. However, this prevents STP from accurately choosing "the best path" in gigabit networks. For a solution, the IEEE decided to modify cost to use a nonlinear scale. The table in the Figure lists the new cost values.