Spanning TreeGlobal Config

spanning-tree vlan [id] priority [value]

Sets the bridge priority for a specific VLAN to influence the root bridge election in Spanning Tree Protocol (STP).

Syntax·Global Config

spanning-tree vlan [id] priority [value]

When to Use This Command

Ensuring a specific switch becomes the root bridge for a VLAN to optimize traffic flow.
Configuring a backup root bridge with a higher priority value for redundancy.
Adjusting priority to force a different switch to become root after a failure.
Troubleshooting STP convergence issues by manually controlling root bridge selection.

Command Examples

Set priority to 4096 for VLAN 10 to make switch root bridge

spanning-tree vlan 10 priority 4096

No output is displayed upon successful configuration. Use 'show spanning-tree vlan 10' to verify the new priority.

Verify priority change with show spanning-tree

show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    4096
             Address     0011.2233.4455
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4096   (priority 4096 sys-id-ext 10)
             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
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 4         128.1    P2p
Gi0/1            Desg FWD 4         128.2    P2p

The 'Root ID' shows priority 4096 and the MAC address of this switch, confirming it is the root. 'This bridge is the root' indicates root status. 'Bridge ID' shows the configured priority (4096) plus the VLAN ID (10) as sys-id-ext. Interface roles: Desg (designated) and FWD (forwarding) are normal for root ports.

Understanding the Output

The 'show spanning-tree vlan [id]' command displays STP information for a specific VLAN. The 'Root ID' section shows the current root bridge's priority and MAC address. If 'This bridge is the root' appears, the local switch is the root. The 'Bridge ID' section shows the local switch's priority (including the VLAN ID as sys-id-ext). Lower priority values (multiples of 4096) are better for becoming root. Interface roles: Root (R) for best path to root, Designated (Desg) for forwarding on segment, Alternate (Altn) or Backup (Back) for blocked ports. State: FWD (forwarding), BLK (blocking), LRN (learning), LIS (listening). Cost is the path cost to root. Watch for unexpected root changes or blocked ports indicating loops.

CCNA Exam Tips

Priority values must be in increments of 4096 (0, 4096, 8192, etc.) on Cisco switches; other values are rejected.

The root bridge is the switch with the lowest bridge ID (priority + MAC address). Changing priority can force a new root election.

In CCNA, know that the default priority is 32768, and setting it to 0 makes the switch likely root.

The 'sys-id-ext' automatically adds the VLAN ID to the priority, so effective priority = configured priority + VLAN ID.

Common Mistakes

Using a priority value not a multiple of 4096 (e.g., 100), which is rejected by IOS.

Forgetting to specify the VLAN ID, causing the command to apply to all VLANs (if supported) or fail.

Setting priority on a switch that already has a lower priority elsewhere, not changing root election.

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 vlan [id] root primary

This command configures a switch as the root bridge for a specific VLAN by setting its bridge priority to 24576 (or lower if another switch has a lower priority), ensuring it becomes the root bridge in the Spanning Tree Protocol (STP) topology.

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