Etherchannel and spanning tree port cost

[v.chow]

How to calculate the port cost /port vlan cost over the etherchannel which consist of 2, 4 and 8 port in the group?

[raymong]

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

[konigl]

I have seen cost of 3 for a two-port GEC on a Cat6509 (normal port cost for a Gigabit Ethernet port is 4; using the formula above gets me 2.35, which must be rounding up to 3.) On the Cat3548XL at the other end, it only shows 4 on the first Gig interface in the GEC; the second Gig interface does not show anything.

I have seen cost of 12 for a two-port FEC on a Cat6509 (normal port cost for a Fast Ethernet port is 19; using the formula above gets me 11.17, which must be rounding up to 12.)

Good formula to keep handy. Thanks for posting it!

Some other vendors (e.g. 3Com) vary the STP cost of a port by duplex setting as well as by 802.3ad link aggregation (standards-based port channeling). Something to keep in mind in mixed-vendor environments.