Spanning Tree Protocol Configuration Guide, Cisco Catalyst IE9300 Rugged Series Switches

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Spanning Tree Protocol Configuration Guide, Cisco Catalyst IE9300 Rugged Series Switches

How a switch becomes the root switch

Updated: March 2, 2026

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This process describes how a switch is elected as the root switch in a network using default spanning-tree settings and how the root switch can be changed by adjusting switch priority.

These stages describe how the root switch is elected.

Summary

In a network with multiple switches, the root switch is elected based on Spanning Tree protocol settings.

  • Switch: Participates in the election process and can become the root switch depending on its priority and MAC address.

The process involves evaluating switch priorities and MAC addresses to determine the root switch, and optionally adjusting priorities to select the ideal root switch.

Workflow

These stages describe how a switch becomes the root switch in a Spanning Tree enabled network.

  1. If all switches in a network are enabled with default Spanning Tree settings, the switch with the lowest MAC address becomes the root switch.
  2. The switch priority of all switches is set to the default value (32,768).
  3. In the example topology, switch A is elected as the root switch because all switches have the default priority (32,768) and switch A has the lowest MAC address.
  4. However, due to traffic patterns, number of forwarding interfaces, or link types, switch A might not be the ideal root switch.
  5. Increase the priority (lower the numerical value) of the ideal switch to make it the root switch. This action triggers a Spanning Tree recalculation and forms a new topology with the ideal switch as the root.

Root port change

A root port change occurs in a Spanning Tree topology when a port with a higher speed link is configured with a higher priority (lower numerical value) than the current root port. As a result, the faster link becomes the new root port.

  • Spanning Tree topology may not always select the fastest link as the root port by default.

  • Changing the port priority on a higher speed link can influence which port becomes the root port.

  • Optimizing port priorities can improve network efficiency by selecting the best available link as the root port.

Root port change in STP topology

When the Spanning Tree topology is calculated using default parameters, the selected path between source and destination may not utilize the fastest available link.

Change the root port to a faster link

If switch B has both a Gigabit Ethernet link and a 10/100 link, and the 10/100 link is currently the root port, network traffic may be more efficient over the Gigabit Ethernet link. Configure the Spanning Tree port priority on the Gigabit Ethernet port to a higher priority (lower numerical value) than the root port. After configuration, the Gigabit Ethernet port becomes the new root port.