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

Alinasirz
Beginner
Beginner

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. 

1 Accepted Solution

Accepted Solutions

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

 

BandwidthOld STP valueNew Long STP value
10 Mbps1002,000,000
100 Mbps19200,000
1 Gbps420,000
N X 1 Gbps310,000
10 Gbps22,000
100 GbpsN/A200
1 TbpsN/A20
10 TbpsN/A2
SpeedResulting Cost
10M100
20M56
30M47
40M41
50M35
60M30
70M26
80M23
  
100M19
200M12
300M9
400M8
500M7
600M6
700M5
800M5
  
1G4
2G3
3G3
4G3
5G3
6G3
7G3
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

View solution in original post

4 Replies 4

Meheretab Mengistu
Rising star
Rising star

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

HTH,
Meheretab

Martin L
VIP
VIP

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


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

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

 

BandwidthOld STP valueNew Long STP value
10 Mbps1002,000,000
100 Mbps19200,000
1 Gbps420,000
N X 1 Gbps310,000
10 Gbps22,000
100 GbpsN/A200
1 TbpsN/A20
10 TbpsN/A2
SpeedResulting Cost
10M100
20M56
30M47
40M41
50M35
60M30
70M26
80M23
  
100M19
200M12
300M9
400M8
500M7
600M6
700M5
800M5
  
1G4
2G3
3G3
4G3
5G3
6G3
7G3
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

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