STP Cost Calculation for Port-Channel (With multiple links)

[Alinasirz]

Hi Guys,

I see that the port-channel with two fast-Ethernet connections has cost of 12. how is it calculated? what is the formula behind that. Also, what if I need to calculate the cost of 4 or 6 Fa connections in port-channel. 

[Meheretab Mengistu]

I do not remember whether I saw a formula to calculate STP costs. However, whenever you add members to Port-channels, you will lower the cost. If you do not want the STP cost to change, you will need to configure STP cost using "spanning-tree cost X".

You can reference in the following link:- https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/spanning-tree-cost.html

HTH,

Meheretab

[Martin L]

based on speed of link. you need to find table with stp costs listed.
not much info but some short and long cost values are listed
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html

[Martin L]

maybe somewhere on this page cost is shown
https://www.cisco.com/c/en/us/tech/lan-switching/spanning-tree-protocol/index.html

but likely you would have to go to IEEE site and look up the standard

[zachrhart@gmail.com · Accepted solution]

Cisco has never published the formula they use to determine STP costs, but the static values are well documented:

Bandwidth	Old STP value	New Long STP value
10 Mbps	100	2,000,000
100 Mbps	19	200,000
1 Gbps	4	20,000
N X 1 Gbps	3	10,000
10 Gbps	2	2,000
100 Gbps	N/A	200
1 Tbps	N/A	20
10 Tbps	N/A	2

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

Ref: https://community.cisco.com/t5/switching/cost-stp-for-etherchannel/td-p/2401310

Ref: https://community.cisco.com/t5/switching/stp-port-cost-and-etherchannel/td-p/2795286

Please remember to vote and mark correct answer if this answers your question!

-Zac