STP Priority

What is STP priority?

Spanning Tree Protocol (STP) is a safety mechanism that prevents loops in networks with multiple switches. If you connect two switches with more than one cable (for redundancy), STP makes sure data only travels one path at a time so it does not circle endlessly and bring the network down.

To do this, STP elects one switch as the "root bridge," the central reference point that all other switches use to decide which paths to keep active and which to block. STP priority is the setting that controls which switch becomes the root bridge. Think of it like choosing a team captain: you want the most capable, centrally located player, not just whoever happened to show up first.

Why it matters

When all switches are left at the default priority (32768), the root bridge is chosen based on the lowest MAC address, which is essentially random. This means your most important, centrally located switch might not be the root bridge. Instead, a small switch in a closet or a desktop switch someone plugged in could end up controlling traffic paths for the entire network.

A poorly chosen root bridge forces traffic to take longer, less efficient paths between devices. Worse, if that accidental root bridge is rebooted or unplugged, the entire network has to reconverge, which can cause seconds or even minutes of downtime while all the switches figure out a new topology. Intentionally setting a lower priority on your core switch prevents all of these problems.

What you can do

• Identify your core switch, the most central and capable switch in your network, and lower its STP priority to 4096 or 8192
• On most managed switches, you can set the priority in the Layer 2 or Spanning Tree section of the admin interface
• On UniFi switches, set the priority in the switch settings under "Spanning Tree"
• Leave secondary or backup switches at a priority like 16384 so they take over predictably if the primary goes down
• Never leave all switches at the default priority of 32768 in a network with more than one switch
• If you only have a single switch, STP priority does not matter since there is only one possible root bridge

What Network Weather shows you

Network Weather checks whether your network switches are all running at the default STP priority, which means the root bridge was chosen arbitrarily instead of intentionally.