12-02-2015 10:13 PM - edited 03-08-2019 02:56 AM
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.
Solved! Go to Solution.
12-03-2015 06:20 AM
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
SpeedA * 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
12-03-2015 06:20 AM
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
SpeedA * 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
12-03-2015 04:15 PM
Thank you very much Peter :-) I had seen one of your earlier posts that was dated a few years ago.
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