STP port cost and Etherchannel

[KnowledgeAcquirer]

I have seen references that two bundled FastEthernet ports into an Etherchannel results in an STP cost of 12.

How is STP cost of 12 derived when the port cost of a single FastEthernet port is 19? --> FastEthernet is 100 Mbps.

I want to grasp and understand this so that I can determine STP cost of an an Etherchannel bundle that has 3,4.5,6,7,8 bundled FastEthernet ports.

Thank you.

[Peter Paluch · Accepted solution]

Hi,

Cisco has never published the formula it uses to recalculate the total EtherChannel bundle bandwidth into the STP cost. It appears to be an exponential function in the format

Speed^A * B = Cost

where A and B are unknown constants.

Nonetheless, to be at least partially helpful, I have tested out all typical bandwidths and I am enclosing a table of the resulting STP costs for your convenience.

Speed	Resulting Cost
10M	100
20M	56
30M	47
40M	41
50M	35
60M	30
70M	26
80M	23
100M	19
200M	12
300M	9
400M	8
500M	7
600M	6
700M	5
800M	5
1G	4
2G	3
3G	3
4G	3
5G	3
6G	3
7G	3
8G	3

Hope it helps.

Best regards,
Peter

[KnowledgeAcquirer]

Thank you very much Peter :-)   I had seen one of your earlier posts that was dated a few years ago.