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Spanning Tree Root Port Calculation

Reviewed by Calculator Editorial Team

In network protocols, the Spanning Tree Protocol (STP) is used to prevent loops in Ethernet networks. One key concept in STP is the root port, which is crucial for determining the network's topology. This guide explains how to calculate the root port in a spanning tree network.

What is a Root Port?

The root port is a designated port on a non-root switch that provides the best path back to the root bridge in the spanning tree topology. It's determined based on the lowest path cost to the root bridge.

Root ports are essential because they help prevent network loops by ensuring that each network segment has only one active path to the root bridge. If multiple paths exist with equal cost, the port with the lowest bridge ID is chosen.

Key Points:

  • Root ports exist only on non-root switches
  • They provide the best path to the root bridge
  • Determined by path cost and bridge ID in case of ties

How to Calculate Root Port

The root port is calculated based on the following criteria:

  1. Path cost to the root bridge (lower is better)
  2. Sender bridge ID (lower is better if path costs are equal)
  3. Sender port ID (lower is better if both path costs and bridge IDs are equal)

Root Port Selection Criteria:

  1. Select the port with the lowest path cost to the root bridge
  2. If path costs are equal, select the port from the bridge with the lowest bridge ID
  3. If both path costs and bridge IDs are equal, select the port with the lowest port ID

Step-by-Step Calculation Process

  1. Identify all possible paths from the non-root switch to the root bridge
  2. Calculate the path cost for each path (sum of all link costs along the path)
  3. Select the path with the lowest path cost
  4. If multiple paths have the same lowest path cost, compare the bridge IDs of the sending bridges
  5. If bridge IDs are also equal, compare the port IDs
  6. The selected port becomes the root port

Example Calculation

Consider a network with the following configuration:

Switch Bridge ID Port Port ID Path Cost
Root Bridge 00:00:00:00:00:01 - - 0
Switch A 00:00:00:00:00:02 1 128.1 4
Switch A 00:00:00:00:00:02 2 128.2 10
Switch B 00:00:00:00:00:03 1 128.1 6

For Switch A:

  1. Port 1 has path cost 4
  2. Port 2 has path cost 10
  3. Port 1 is selected as root port because it has the lower path cost

For Switch B:

  1. Port 1 has path cost 6
  2. Since there's only one port, it's automatically selected as root port

Frequently Asked Questions

What happens if multiple ports have the same path cost?

If multiple ports have the same path cost, the port from the bridge with the lowest bridge ID is selected. If bridge IDs are also equal, the port with the lowest port ID is chosen.

Can a root port change if the network topology changes?

Yes, root ports can change if the network topology changes, such as when a link fails or a new switch is added. The spanning tree protocol will recalculate the topology and select new root ports as needed.

What is the difference between a root port and a designated port?

A root port provides the best path back to the root bridge, while a designated port is used to forward traffic on a LAN segment. Root ports exist only on non-root switches, while designated ports can exist on any switch.

How does the path cost affect root port selection?

The path cost is the sum of all link costs along the path to the root bridge. The port with the lowest path cost is selected as the root port, as it provides the most efficient path to the root bridge.