This command will prevent a switch that has lost its link to the root switch from becoming root when the root is still running. If switches receive an inferior BPDU from a switch, backbonefast will send them the proper information for reaching the root.
In the below example we will configure backbonefast on all switches. After several link failures, we can see that SW4 is correcting SW2’s BPDUs.
SW1(config)#spanning vlan 1-4094 root primary
SW4(config-if-range)#do show spanning back