Skip to content

Spanning Tree Protocol (STP) Guide

Learn how to configure Spanning Tree Protocol to prevent network loops in your switch topology.

What is STP?

Spanning Tree Protocol (IEEE 802.1D) prevents Layer 2 loops in networks with redundant switch connections. Without STP, broadcast frames would loop indefinitely, causing a broadcast storm that brings down the network.

How STP Works

  1. Root Bridge Election - Switches elect one switch as the root based on Bridge ID (priority + MAC)
  2. Path Calculation - Each switch calculates the best path to the root
  3. Port Role Assignment - Ports are assigned roles (root, designated, alternate)
  4. Port State Management - Blocking ports prevent loops while maintaining redundancy

STP Port States

State Description
Disabled Port is administratively down
Blocking Port blocks user traffic but receives BPDUs
Listening Transitional state (not used in simulation)
Learning Learning MAC addresses but not forwarding
Forwarding Port forwards all traffic normally

STP Port Roles

Role Description
Root Port Best path to root bridge (one per non-root switch)
Designated Port Best port for a segment to reach root
Alternate Port Backup path, currently blocking
Disabled Port is down or STP disabled on it

Enabling STP

Open a switch terminal and enable STP:

spanning-tree enable

This initializes STP with default values: - Bridge Priority: 32768 - Hello Time: 2 seconds - Max Age: 20 seconds - Forward Delay: 15 seconds

Viewing STP Status

Basic Status

show spanning-tree

Output shows: - Whether STP is enabled - Bridge ID and priority - Root bridge information - Root path cost - Timer values

Per-Port Status

show stp interface

Shows for each port: - Role (root/designated/alternate/disabled) - State (forwarding/blocking/disabled) - Path cost - Port priority

Detailed Port Info

show stp detail

Configuring Root Bridge

To make a switch the root bridge, lower its priority:

spanning-tree priority 4096

Valid priorities: 0 to 61440 (multiples of 4096) Lower priority = more likely to become root

Priority Values Guide

Priority Typical Use
0 Forced root (highest priority)
4096 Primary root
8192 Secondary root
32768 Default

Configuring Port Cost

Path cost affects root path selection. Lower cost = preferred path.

Enter interface configuration mode first: 

interface GigabitEthernet0/0
spanning-tree cost 10

Default Path Costs (IEEE 802.1D-2004)

Speed Cost
10 Gbps 2
1 Gbps 4
100 Mbps 19
10 Mbps 100

Configuring Port Priority

Port priority is used as a tiebreaker when multiple ports have equal cost:

interface GigabitEthernet0/0
spanning-tree port-priority 64

Valid values: 0-255 (default: 128)

Disabling STP

To disable STP (not recommended in redundant topologies):

spanning-tree disable

⚠️ Warning: Disabling STP in a topology with loops will cause broadcast storms!

Force Reconvergence

To manually trigger STP recalculation after topology changes:

spanning-tree reconverge

Example: Triangle Topology

        [SW1] (Root)
       /      \
      /        \
   [SW2]------[SW3]
               ^ blocked

Configuration

SW1 (Root Bridge): 

spanning-tree enable
spanning-tree priority 4096

SW2: 

spanning-tree enable

SW3: 

spanning-tree enable

After convergence: - SW1 is root (lowest priority) - SW2 and SW3 have root ports pointing to SW1 - One port on the SW2-SW3 link is blocking

Troubleshooting

All Ports Blocking

  • Check if spanning-tree enable was run
  • Run spanning-tree reconverge to force recalculation
  • Verify connections are up

Unexpected Root Bridge

  • Check bridge priorities with show spanning-tree
  • Lower priority on desired root switch
  • Remember: lower priority wins

Traffic Not Flowing

  • Check port states with show stp interface
  • Verify blocking ports are expected
  • Ensure at least one path exists through forwarding ports

Topology Changes Not Detected

  • Run spanning-tree reconverge manually
  • Check that all switches have STP enabled
  • Verify connections are properly established
Command Description
spanning-tree enable Enable STP
spanning-tree disable Disable STP
spanning-tree priority <value> Set bridge priority
spanning-tree cost <value> Set port cost
spanning-tree port-priority <value> Set port priority
spanning-tree reconverge Force recalculation
show spanning-tree Display STP status
show stp interface Display port states
show stp detail Display detailed port info