## How is STP path cost calculated?

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 path cost?

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.

## What is the default spanning tree path cost of a 1 Gbps link?

IEEE 802.1t Path Cost Parameter Values

Link Speed Recommended Value Range
1 Gbps 20.000 1-200.000.000
10 Gbps 2.000 1-200.000.000
100 Gbps 200 1-200.000.000
1 Tbps 20 1-200.000.000

## How do I change the spanning tree cost on an interface?

By changing the STP port costs with the command spanning tree [vlan vlan-id] cost cost, you can modify the STP forwarding path. You can lower a path that is currently an alternate port while making it designated, or you can raise the cost on a port that is designated to turn it into a blocking port.

## 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.

## What is the STP port cost for a 10 Mbps Ethernet connection?

As shown in Table 3-1, 10 Gbps Ethernet ports have a port cost of 2, 1 Gbps Ethernet ports have a port cost of 4, 100 Mbps Fast Ethernet ports have a port cost of 19, and 10 Mbps Ethernet ports have a port cost of 100.

## How do you calculate spanning tree?

Theorem: A graph is connected iff it has a spanning tree. Proof: If a graph is connected, we can identify a cycle and remove an edge from it: it will still be connected. We can continue this until no cycles remain. The result is a spanning tree.

## What are the three criteria that a switch will determine a port when it has multiple equal cost paths?

When a switch has multiple equal-cost paths to the root bridge, the switch will determine a port using the following criteria: Lowest sender BID. Lowest sender port priority. Lowest sender port ID.

## What is the purpose of spanning tree Protocol?

STP prevents loops in layer 2 networks that have redundant paths. Loops in layer 2 networks can result in duplicate frames and switches learning the same MAC addresses on multiple interfaces. To prevent loops, STP calculates a single path through the network and then blocks any redundant paths.

## What are spanning tree ports cost, spanning tree port?

The following table lists the Port Cost value for different bandwidths. As we can see from the above diagram, Switch 4 has two paths to reach the Root Switch (Switch 1). The accumulated Spanning Tree Path Cost Value via the left path is 23 (19 + 4) and the accumulated Spanning Tree Path Cost Value via the right path is 8 (4 + 4).

## Do you need a root switch for spanning tree?

• A unique root switch is elected for the spanning tree network topology. • A designated switch is elected for every switched LAN segment.

## What happens when a spanning tree becomes unreachable?

If one network segment becomes unreachable, or if spanning tree costs change, the spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating a standby path.

## Which is the spanning-tree cost command in interface mode?

The command is “spanning-tree cost 1” in interface mode. Is there any explanation as why only two of the vlans change the cost? I hope to be clear on the scenario.

How is STP path cost calculated? 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…