Path Cost is used by the Spanning Tree Algorithm to determine the best path between devices.
Therefore, lower values should be assigned to ports attached to faster media, and higher values assigned to ports with slower media.
By default, the system automatically detects the speed and duplex mode used on each port, and configures the path cost according to the values shown below.
*The path cost of the STP is not configured by pathcost method short or long.
User can configure the spanning tree path cost for the specified interface by following command.
[CLI Command]
spanning-tree cost {cost}
cost - The path cost for the port.
(Range: 0 for auto-configuration, 1-65535 for short path cost method, 1-200,000,000 for long path cost method)
Calculate the spanning tree path cost on a port-channel.
1. Active Eth1/1 for port channel.
Console#show interfaces brief Interface Name Status PVID Pri Speed/Duplex Type Trunk --------- ----------------- --------- ---- --- ------------- ------------ ----- Eth 1/ 1 Up 1 0 Auto-1000full 1000BASE-T 1
The spanning tree path cost on Trunk 1 is 5000.
Console#show spanning-tree brief Interface Pri Designated Designated Oper STP Role State Oper Bridge ID Port ID Cost Status Edge --------- --- --------------------- ---------- -------- ------ ---- ----- ---- Trunk 1 128 32768.8CEA1B8AC667 128.57 5000 EN ROOT FWD No
The spanning tree path cost for Trunk 1 is 10000 (1G) / 2 = 5000 (Trunk).
The spanning tree path cost on Trunk 1 is 5000 (Trunk) / 1 = 5000.
2. Active Eth1/1 & Eth1/2 for port channel.
Console#show interfaces brief Interface Name Status PVID Pri Speed/Duplex Type Trunk --------- ----------------- --------- ---- --- ------------- ------------ ----- Eth 1/ 1 Up 1 0 Auto-1000full 1000BASE-T 1 Eth 1/ 2 Up 1 0 Auto-1000full 1000BASE-T 1
The spanning tree path cost on Trunk 1 is 2500.
Console#show spanning-tree brief Interface Pri Designated Designated Oper STP Role State Oper Bridge ID Port ID Cost Status Edge --------- --- --------------------- ---------- -------- ------ ---- ----- ---- Trunk 1 128 32768.8CEA1B8AC667 128.57 2500 EN ROOT FWD No
The spanning tree path cost on Trunk 1 is 5000 (Trunk) / 2 = 2500.
3. Active Eth1/1 & Eth1/2 & Eth1/3 for port channel.
Console#show interfaces brief Interface Name Status PVID Pri Speed/Duplex Type Trunk --------- ----------------- --------- ---- --- ------------- ------------ ----- Eth 1/ 1 Up 1 0 Auto-1000full 1000BASE-T 1 Eth 1/ 2 Up 1 0 Auto-1000full 1000BASE-T 1 Eth 1/ 3 Up 1 0 Auto-1000full 1000BASE-T 1
The spanning tree path cost on Trunk 1 is 1666.
Console#show spanning-tree brief Interface Pri Designated Designated Oper STP Role State Oper Bridge ID Port ID Cost Status Edge --------- --- --------------------- ---------- -------- ------ ---- ----- ---- Trunk 1 128 32768.8CEA1B8AC667 128.57 1666 EN ROOT FWD No
The spanning tree path cost on Trunk 1 is 5000 (Trunk) / 3 = 1666.
4. Active Eth1/1 & Eth1/2 & Eth1/3 & Eth1/4 for port channel.
Console#show interfaces brief Interface Name Status PVID Pri Speed/Duplex Type Trunk --------- ----------------- --------- ---- --- ------------- ------------ ----- Eth 1/ 1 Up 1 0 Auto-1000full 1000BASE-T 1 Eth 1/ 2 Up 1 0 Auto-1000full 1000BASE-T 1 Eth 1/ 3 Up 1 0 Auto-1000full 1000BASE-T 1 Eth 1/ 4 Up 1 0 Auto-1000full 1000BASE-T 1
The spanning tree path cost on Trunk 1 is 1250.
Console#show spanning-tree brief Interface Pri Designated Designated Oper STP Role State Oper Bridge ID Port ID Cost Status Edge --------- --- --------------------- ---------- -------- ------ ---- ----- ---- Trunk 1 128 32768.8CEA1B8AC667 128.57 1250 EN ROOT FWD No
The spanning tree path cost on Trunk 1 is 5000 (Trunk) / 4 = 1250.
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