Spanning Tree

Port States

Sections:

Overview:

  • The port states in Spanning Tree define the role of a port in relation to its position in the Spanning Tree topology
  • These port states help ensure safe transitions, prevent loops, and allow network reconvergence after topology changes

Summary:

  • Blocking state - blocking state only receives BPDUs and does not forward traffic
  • Listening & Learning states - states only receive and forward BPDUs but does forward traffic
  • Learning state - learning state begins populating its MAC address table from incoming frames 
    • Forwarding state - forwarding state sends traffic in addition to receiving and forwarding BPDUs
      • 802.1D Port States:
          • Disabled
          • Blocking
          • Listening
          • Learning
          • Forwarding
        • 802.1W Port States:
            • Discarding
            • Learning
            • Forwarding

          Disabled State

          Overview:

          • The disabled state is when a port is administratively shut down and not participating in the Spanning Tree domain
          • A port in the disabled state does not send or receive any traffic including BPDUs
          • This state is used when a port is intentionally turned off and should not participate in the STP domain

          Blocking State

          Overview:

          • A port in the blocking state does not forward traffic and only listens for BPDUs to detect for any changes in the network topology
          • A port in the blocking state remains in this state to prevent loops on the network
          • Blocking a port helps ensure that only one path is active between two switches, while backup paths remain blocked until needed

          Listening State

          Overview:

          • The listening state is the phase where a port is checking for BPDUs before transitioning into either a Learning or Forwarding state
          • A listening state port does not forward traffic and only listens for BPDUs
          • The listening state allows a port to gather network topology information before any learning or forwarding of data begins

          Learning State

          Overview:

          • The learning state is when a port begins to learn MAC addresses but still does not forward traffic
          • This state is essential to allow ports to effectively begin forwarding packets immediately after transitioning into the forwarding state 

          Forwarding State

          Overview:

          • The forwarding state is when a port actively forwards frames to and from the network
          • The forwarding state learns MAC addresses and forwards data
          • The forwarding state represents the port's active participation in the network 

          Discarding State

          Overview:

          • The discarding state in 802.1w replaces the older states from 802.1D including the Blocking, Listening, and Disabled states
          • The discarding state is a non-forwarding state where a port does not forward traffic and is a simplified state used to prevent loops and ensure faster convergence when network changes occur on the network