spanning-tree port-priority [value]
Sets the port priority value for a specific interface to influence which port becomes the root port in a Spanning Tree topology.
spanning-tree port-priority [value]When to Use This Command
- Manually selecting a preferred root port on a non-root bridge to control traffic flow.
- Load balancing traffic across redundant links by adjusting port priorities.
- Ensuring a specific high-bandwidth link becomes the root port for faster convergence.
- Preventing a slow or unreliable link from being elected as the root port.
Command Examples
Setting port priority to 64 on GigabitEthernet0/1
spanning-tree port-priority 64Switch(config-if)# spanning-tree port-priority 64 Switch(config-if)# end Switch# show spanning-tree interface gigabitEthernet 0/1 Vlan Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- VLAN0001 Root FWD 4 64.1 P2p VLAN0010 Desg FWD 4 64.1 P2p
The 'spanning-tree port-priority 64' command sets the priority to 64 for this interface. The 'show spanning-tree interface' output shows the port role (Root or Desg), state (FWD), cost, and the priority.nbr (64.1). The priority value (64) is lower than default (128), making this port more likely to be selected as root port.
Verifying port priority change on FastEthernet0/1
spanning-tree port-priority 32Switch(config-if)# spanning-tree port-priority 32
Switch(config-if)# end
Switch# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0011.2233.4455
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.2233.4455
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 32.1 P2p
Fa0/2 Desg FWD 19 128.2 P2pAfter setting priority to 32 on Fa0/1, the 'show spanning-tree vlan 1' output shows Fa0/1 with priority 32.1 (priority 32, port number 1). The lower priority makes it more favorable, but since this bridge is root, both ports are designated. The priority influences which port becomes root on downstream switches.
Understanding the Output
The 'show spanning-tree' or 'show spanning-tree interface' command displays the spanning-tree state per VLAN. The 'Pri.Nbr' column shows the port priority (first number) and port number (second number). Lower priority values make a port more likely to be elected as the root port. In the output, 'Role' indicates the port role (Root, Desg, Altn, Back), and 'Sts' shows the state (FWD, BLK, LRN, LSN). A priority of 128 is default; values from 0 to 240 in increments of 16 are valid. If you see a port with a lower priority than others on the same switch, that port is preferred for root port selection. Watch for misconfigurations where a port with a high priority (e.g., 240) might be blocked unnecessarily.
CCNA Exam Tips
CCNA exam tip: Port priority values must be multiples of 16 (0, 16, 32, ..., 240).
CCNA exam tip: The default port priority is 128; lowering it makes the port more likely to become root port.
CCNA exam tip: Port priority is used as a tie-breaker when path costs are equal.
CCNA exam tip: Remember that port priority is configured per interface, not globally.
Common Mistakes
Mistake: Setting port priority to a non-multiple of 16 (e.g., 50) — the IOS will reject it.
Mistake: Confusing port priority with bridge priority — port priority is per interface, bridge priority is per switch.
Mistake: Forgetting to apply the command in interface configuration mode.
Related Commands
show spanning-tree
Displays the Spanning Tree Protocol (STP) state and configuration for all VLANs or a specific VLAN, used to verify root bridge, port roles, and STP topology.
spanning-tree cost [value]
Sets the path cost for a specific interface in Spanning Tree Protocol (STP) calculations, overriding the default cost based on interface speed.
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