NetPro member raymong (Raymond Ng) posted the following formula here on Jul 21, 2003. I’m not sure what his original source was, but this formula provides results close enough to values I’ve seen that I use it on a regular basis.
“Sum of indiv costs/(N^1.70) , where N is number of channeling ports.
For example, if you have a 4-port channel, each with a individual cost of 100, it would be:
100+100+100+100/(4^1.70) = 37.8929”
If the result is not a whole number, I round up to the next nearest integer: so 37.8929 becomes 38. It’s not exactly the same as the observed value for a 4-port EtherChannel in the table Milan provided the link to; but it’s close enough to work.
The calculations for Fast EtherChannel (with round-off to next higher whole number) are an identical match with the observed values in the table: a 4x100Mbps FEC yields (19+19+19+19)/(4^1.70)=7.1996, or rounded up, 8.
The usefulness of the formula diminishes as you get into anything bigger than 2x1Gbps GEC using IEEE 802.1D path costs.
Hope this helps.
