Contents
What is path cost in STP?
The path cost is the metric STP uses to calculate the shortest path to the elected root bridge. The path cost is based on the speed of the bridge port interface. You can confirm the path cost method being used on your Cisco switch with “show spanning-tree summary” command.
How is spanning tree path cost calculation?
It will use its 1000 Mbit interface as its root port (shortest path to the root bridge is 19+19+4=42). SW3 will forward BPDUs to SW4. The root path cost field will be 100. SW4 receives a BPDU from SW2 with a root path cost of 19….Spanning-Tree Cost Calculation.
| Bandwidth | Cost |
|---|---|
| 10 Mbit | 100 |
| 100 Mbit | 19 |
| 1000 Mbit | 4 |
What is STP priority?
When a loop occurs in a network topology, spanning tree can use the port priority value for the ports to decide which port must be put in forwarding state. The port priority is only used to determine the topology if the loop in the network cannot be resolved using bridge IDs or path cost.
How does STP calculste the root bridge cost?
06-16-2009 11:38 AM as Toshi has explained STP calculstes the cost to the root bridge but it does it by adding the local port cost that receives the BPDU to the path cost in the BPDU. notice that only port-id changes from port to port.
How is the cost of a link calculated in STP?
For STP, a switch looks at the cost on its interface for the link because the cost of a link is the same on both ends, there is only one cost for the link. You only add costs for multiple links to the root.
What is the spanning tree path cost value?
Spanning Tree Path Cost value can be defined as the accumulated port costs from a Switch (other than the Root Bridge (Switch)) to reach the Root Switch. When a switch receives a Bridge Protocol Data Unit (BPDU) in its port, it increments the path cost with the cost of the incoming port.