Spanning Tree

Port Roles

Sections:

Overview:

  • Spanning Tree port roles play a critical role in maintaining a loop-free topology, ensuring network redundancy, and optimizing traffic flow
  • By assigning port roles, STP helps manage the network's behavior and automatically adapts to topology changes, ensuring stability, resilience, and efficient use of network resources

Spanning Tree Port Role Version Comparisons 

Newer versions of Spanning Tree such as 802.1w introduced new terminology for the same port roles. 

The terminology change from 'Blocking' to 'Alternate' reflects a shift in how the Spanning Tree protocol standard handles redundancy and failover.

Below is a list of the port types that were introduced in each Spanning Tree standard:

  • 802.1D
    • Designated
    • Root
    • Blocking
  • 802.1W
    • Designated
    • Root Port
    • Alternate (Backup Root Port)
    • Backup (Backup Designated Port)

Designated Ports

Overview:

  • A designated port is a port on a switch that is responsible for forwarding traffic towards the root bridge
  • The designated port on a switch has the lowest cost path to the root bridge for a particular network segment
  • There is one designated port per network segment between switches
  • The designated port ensures that there is a single efficient forwarding path for each network segment and helps prevent loops by ensuring only one path is active at any given time
  • The designated port is always in the forwarding state

By default, all ports on the Root Switch have the designated role.

Root Ports

Overview:

  • The root port is the port on a switch that provides the best path (lowest path cost) to reach the root bridge
  • The root port is the port that receives BPDUs and traffic destined for the root bridge
  • A non root switch can only consist of only one root port and chooses the port with the lowest path cost to the root bridge
  • The root port allows a switch to know the path to the root bridge and ensures the most efficient path is used for communications with the root bridge
  • The root port is always in a forwarding state

By default, all non-root switches have only one root port assigned to one of its interfaces.

Alternate Ports

Overview:

  • An alternate port is a backup port that is ready to transition to the root port if the current root port fails
  • An alternate port serves as an alternate path to the root bridge and is placed in a blocking state until needed
  • Alternate ports provide network redundancy by ensuring that backup paths are available for the root bridge reducing the risk of network outages due to link failures
  • The alternate port is always in a non forwarding state

Blocking Ports

Overview:

  • The blocking port role is crucial for preventing Spanning Tree loops on the network
  • A blocking port is any port that does not forward traffic because it is not part of the active topology in the Spanning Tree domain
  • A port is placed into the blocking state if it is not the root or designated port
  • In a blocking state, a port listens for BPDUs but does not forward traffic or BPDUs to other switches
  • The blocking port is kept in this state until there is a network change due to a link failure that makes it necessary to transition the port to a forwarding state
  • The blocking port is always in a non forwarding state

Backup Ports

Overview:

  • A backup port is a port that provides a backup path to a segment where the designated port already exists 
  • Backup ports are only common in situations when two switches are connected to the same segment with the use of a layer 1 hub device, creating an alternate backup path
  • Backup ports are placed in a blocking state but can transition to forwarding if the current designated port becomes unavailable 
  • Backup ports are always in a non forwarding state