Backbonefast: The Ultimate Solution for Seamless Connections
Introduction
Backbonefast is a Cisco proprietary feature that was designed to prevent loops in Cisco switches if redundant paths are available. Normally, we use backbonefast in legacy IEEE 802.1D STP protocol to improve its features like convergence time as well as a loop prevention mechanism. Normally, it works in indirect link failure. However, advancements in STP as rapid spanning tree protocol (RSTP) and multiple spanning tree protocol (MST) protocols are now adopted by enterprises to reduce time, as well as minimize configuration for link failure.
How Backbonefast Works?
We will understand backbonefast through the following topology;

The topology consists of three swathes: switch1, switch2, and switch3. Switch1 is the root.
Suppose switch1 is the root switch. interface fast Ethernet 0/2 of switch2 is blocking, while switch3’s fast Ethernet 0/3 is the root port. If Fa 0/3 of switch3 suddenly breaks down, then switch3 will send less precedent or inconsistent BPDUs through Fa 0/2 to switch2. As the switch2 is directly connected to the root switch1 and is already receiving high precedence BPDUs from the root switch1 through port Fast ethernet 0/1.
When the switch2 receives less precedence BPDUs from the switch3, switch2 will generate a root link query (RLQ) and will send it to the root switch1. The purpose of the RLQ is to ask from switch 1, whether you are my root switch or not. After confirming root switch1, it will notice that there is a problem with switch3’s link.
After confirming that the root switch exists, it will listen to the less precedent BPDUs from switch3.
When we don’t enable bodyguard on a switch, then switch 2 will ignore the BPDU for 20 seconds, which is known as the Max age timer. After 20 seconds, it will wait for listening (15 sec) and learning (15 sec) plus 2 seconds, which is a total delay of 52 seconds.
But, if we enable backbonefast on the switches, then the switch will skip the Max age timer (20 seconds), and now will wait for 30 seconds (listening + learning).
Backbonefast helps in preventing loops as well as reducing convergence time.
It is necessary to configure backbonefast on all the switches in the global configuration mode. If we don’t configure it on any switch, then it may not recognize RLQ (Root Link Query) requests, leading to inconsistencies in the STP topology.
The following command enables it on the switch;
switch(config)#spanning-tree backbonefast