Unlike UplinkFast that’s used to determine and quickly fix link failures on the local switch,
another Cisco-propriety STP extension called BackboneFast is used for speeding up convergence
when a link that’s not directly connected to the switch fails. If a switch running BackboneFast
receives an inferior BPDU from its designated bridge, it knows that a link on the path
to the root has failed. Just to make sure you’re clear on this: an inferior BPDU is one that lists
the same switch for the root bridge and the designated bridge.
Switch(config)#spanning-tree backbonefast ?
BackboneFast is enabled
BackboneFast statistics
-----------------------
Number of transition via backboneFast (all VLANs) : 0
Number of inferior BPDUs received (all VLANs) : 2
Number of RLQ request PDUs received (all VLANs) : 0
Number of RLQ response PDUs received (all VLANs) : 1
Number of RLQ request PDUs sent (all VLANs) : 1
Number of RLQ response PDUs sent (all VLANs) : 0
S2(config)#