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Beginner

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

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.

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Accepted Solutions
Beginner

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

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

-Zac

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Enthusiast

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

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

Enthusiast

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

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
Enthusiast

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

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
Beginner

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

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