Jeff Huston

There are many mechanisms out there that, for routing purposes, assign a cost to a particular route or connection in order to evaluate the best path to take in getting data from point A to point B.  The following is a basic calculation that takes into account bandwidth and latency in attempt to provide a reasonable cost associated with a particular path.

Cost = 1 + ROUND ( 1024 / Log (bandwidth * 1024) * (latency / 20), 0)

Bandwidth is the reported maximum bandwidth available on the link.  This should not include “burstable” bandwidth.  Bandwidth here is measured in Mbit/second; adjust a Kbit/second value by dividing it by 1000.

Latency is the average latency in milliseconds tested on the link.  Use a PING command with a sample set of at least 20 attempts, taking the average value seen.

The ROUND statement above is based on the Excel formula, but essentially it is rounding to zero decimal places (a whole integer).

For example, a T1 link across the country will have a bandwidth of 1.544 Mbit and an average latency of 78 ms.  The cost here would be 1249.  Increasing the bandwidth to a T3 (44.736 Mbit) would change the cost to 858.

A gigabit link between buildings with an average latency of 6 ms would have a cost of 52.

A 10 Mbit link to a DR site with a latency of 18 ms would have a cost of 231.