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Legacy STP - Uplinkfast Backbonefast - Why not in RSTP

 UplinkFast and BackboneFast are two optimizations used in Legacy STP (802.1D) to improve convergence times during certain network failures, particularly in large topologies. Here’s a breakdown of both:

UplinkFast:

  • Purpose: UplinkFast was designed to improve recovery time when a direct uplink to the root bridge fails, especially in access layer switches with redundant uplinks to the distribution layer.
  • How it Works: If the primary uplink (Root Port) fails, UplinkFast immediately switches to a backup uplink (an Alternate Port) without waiting for the normal spanning tree convergence process. This allows the switch to rapidly restore connectivity by immediately transitioning the Alternate Port to the Forwarding state.
  • Use Case: Primarily used in access switches with redundant uplinks to quickly restore connectivity when the main uplink fails.
  • Convergence Time: UplinkFast can achieve sub-second recovery times for uplink failures.

BackboneFast:

  • Purpose: BackboneFast speeds up the recovery of indirect link failures (failures that occur elsewhere in the network, not directly on the local switch), such as when a root port on an upstream switch fails.
  • How it Works: Normally, an indirect failure would require waiting for the Max Age timer (20 seconds by default) before recalculating the spanning tree. BackboneFast bypasses this delay by using inferior BPDUs (received from downstream switches). Upon detecting an inferior BPDU (which suggests a link failure on the path to the root), the switch immediately checks whether the root is still reachable through other paths and recalculates the tree if necessary.
  • Use Case: Used throughout the network to improve convergence in response to failures anywhere in the spanning tree topology.
  • Convergence Time: BackboneFast can reduce recovery time to a few seconds, bypassing the Max Age timer.

Were UplinkFast and BackboneFast used by Legacy STP (802.1D)?

Yes, UplinkFast and BackboneFast were introduced specifically to address the slow convergence times of Legacy STP (802.1D). These optimizations were optional features that could be enabled to reduce downtime caused by link failures in large networks.

Does RSTP natively support UplinkFast and BackboneFast functionalities?

Yes, RSTP (802.1w) natively supports the functionalities of both UplinkFast and BackboneFast through the use of Alternate and Backup Ports. These ports provide built-in mechanisms to quickly recover from link failures:

  • Alternate Port: A port that offers an alternative path to the root bridge (similar to UplinkFast).
  • Backup Port: A port that offers a backup connection to the same segment (similar to a local redundancy mechanism).

In RSTP, these mechanisms are integrated into the protocol itself, ensuring fast convergence without the need for additional configuration as was required with UplinkFast and BackboneFast in Legacy STP. This makes RSTP much faster and more efficient compared to 802.1D.

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