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

Spanning Tree interface states

Updated: March 2, 2026

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Lists the five states that layer 2 interfaces can exist in when using Spanning Tree protocol.

Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can occur at varying times and locations within a switched network. An interface that transitions directly from nonparticipation in Spanning Tree topology to the forwarding state can create temporary data loops. Interfaces must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for frames forwarded using the old topology.

Each layer 2 interface on a switch using Spanning Tree exists in one of these states:

State

Description

Blocking

The interface does not participate in forwarding frames.

Listening

This is the first transitional state after blocking, when Spanning Tree determines that the interface should participate in forwarding frames.

Learning

The interface prepares to participate in forwarding frames.

Forwarding

The interface forwards frames.

Disabled

The interface is not participating in Spanning Tree because it is shut down, there is no link, or no Spanning Tree instance exists.

Interface state transitions

These stages describe how an interface transitions through Spanning Tree states.

  • From initialization to blocking.

  • From blocking, the interface transitions to listening or to disabled.

  • From listening, the interface transitions to learning or to disabled.

  • From learning, the interface transitions to forwarding or to disabled.

  • From forwarding, the interface can move to disabled.

When you power up the switch, the spanning tree feature is enabled by default. Every interface in the switch, VLAN, or network goes through the blocking state, then transitions through the intermediate listening and learning states. The Spanning Tree protocol stabilizes each interface in either the forwarding or blocking state.

How forwarding state transitions work

The Spanning Tree algorithm places a layer 2 interface in the forwarding state. When this occurs, the interface goes through a specific sequence of state transitions.

Summary

These actors and components participate in this process:

  • Spanning Tree algorithm: Manages the state transitions of Layer 2 interfaces.

  • Layer 2 interface: Undergoes state changes as directed by the Spanning Tree protocol.

  • Forward delay timer: Controls timing for transitions between states.

  • Switch: Learns end-station location information during the learning state.

This process summarizes how a layer 2 interface transitions through listening, learning, and forwarding states under the control of the Spanning Tree algorithm.

Workflow

These stages describe the transitions a layer 2 interface undergoes when moving to the forwarding state.

  1. The interface enters the listening state while Spanning Tree waits for protocol information to determine if it should move to the blocking state.
    • The interface is in the listening state.
    • Spanning Tree waits for protocol information.
  2. Spanning Tree moves the interface to the learning state and resets the forward delay timer while waiting for the timer to expire.
    • The interface transitions to the learning state.
    • The forward delay timer is reset.
  3. In the learning state, the interface continues to block frame forwarding as the switch learns end-station location information for the forwarding database.
    • Frame forwarding remains blocked.
    • The switch learns end-station locations for the forwarding database.
  4. When the forward-delay timer expires, Spanning Tree moves the interface to the forwarding state, enabling both learning and frame forwarding.
    • The forward-delay timer expires.
    • The interface enters the forwarding state.
    • Both learning and frame forwarding are enabled.

Interface state functions

This topic describes the functions performed by layer 2 interfaces in each Spanning Tree state: blocking, listening, learning, forwarding, and disabled.

Blocking state

After initialization, a BPDU is sent to each switch interface. A switch initially functions as the root. It remains in this role until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is the root or root switch. If there is only one switch in the network, no exchange occurs. The forward delay timer expires, and the interface moves to the listening state.

An interface always enters the blocking state after switch initialization.

An interface in the blocking state performs these functions:

  • Discards frames received on the interface

  • Discards frames switched from another interface for forwarding

  • Does not learn addresses

  • Receives BPDUs

Listening state

The listening state is the first state a layer 2 interface enters after the blocking state. The interface enters this state when the Spanning Tree decides that the interface should participate in frame forwarding.

An interface in the listening state performs these functions:

  • Discards frames received on the interface

  • Discards frames switched from another interface for forwarding

  • Does not learn addresses

  • Receives BPDUs

Learning state

When a layer 2 interface enters the learning state from the listening state, it prepares to participate in frame forwarding.

An interface in the learning state performs these functions:

  • Discards frames received on the interface

  • Discards frames switched from another interface for forwarding

  • Learns addresses

  • Receives BPDUs

Forwarding state

When a layer 2 interface enters the forwarding state from the learning state, it begins forwarding frames.

An interface in the forwarding state performs these functions:

  • Receives and forwards frames received on the interface

  • Forwards frames switched from another interface

  • Learns addresses

  • Receives BPDUs

Disabled state

When a layer 2 interface enters the disabled state, it does not participate in frame forwarding or the Spanning Tree and becomes nonoperational.

A disabled interface performs these functions:

  • Discards frames received on the interface

  • Discards frames switched from another interface for forwarding

  • Does not learn addresses

  • Does not receive BPDUs