|
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.
|
|