STP BackboneFast

                            BackboneFast

UplinkFast provides faster convergence process if directly-connected local port fail. inorder to handle indirect failures , BackboneFast was introduced in the STP technology. Let say link between SW1n SW3 fails, because of this failure, eventually Sw5 has to recalculate the STP topology and learn path through Sw4 to reach root bridge , how ever Sw5 has to wait Max Age Timer (20sec) before purging SW3's superior BPDU and accept inferior BPDU  from SW4.

So what BackboneFast feature does is it simply bypass the Max Age timer on Sw5 and immediately start accepting inferior BPDU from sw4. And Blocked port on Sw5 gradually transitioned into forwarding state and thus its essentially reduced total convergence time from 50 sec to 30 sec during Indrect failure. This is accomplished by sending out Root Link Query(RLQ) by Sw5,

Root Bridge will response to RLQ with RLQ reply in two ways.

1. If RLQ reply is received on root port (Sw5)then switch assumes that root port is stable and don't act further .

2. If RLQ reply from root bridge is received on Non Root Port then switch (Sw5) knows that it path to root bridge has been failed n it's own Max Age Timer expired immediately and start electing Root port again.

BackboneFast is enabled globally and it should enable on all switches participating in STP topology. 

Switch(config)#spanning-tree backbonefast