Reducing the Indirect Root Port Down Time of STP by BackboneFast

BackboneFast is to start recalculate immediately for indirect failures.
What does that even mean?? Let's see..

In the topology, SW-A is the Root.

So E0/1 of SW-B & E0/2 of SW-C are Root Ports..

E0/3 of SW-C is the Alternate Port..

All are Fast Ethernet ports..


In the above topology, because SW-B has all ports in Forwarding state, an indirect failure is a failure of the link between SW-A & SW-B.. In other words it is a indirect link from SW-C's point of view which maintains the Alternate Port..

Let's see what happens when a Root Port is down..
I have enabled debug spanning-tree events on SW-C..

Removing the SW-A to SW-B link..
(click on images to see in full size)

What happened is that when SW-B loses the link to the Root Bridge, it thinks it is the Root Bridge and sends Inferior BPDUs towards SW-C's Alternate port..
SW-C's Alternate Port will go from Blocking > Listening > Learning > Forwarding in the normal manner which will take 50 seconds by default.

Now let's connect the cable again and configure BackboneFast..
This is a global command and should be enabled on all switches in order to work..

SW-A(config)#spanning-tree backbonefast
SW-B(config)#spanning-tree backbonefast
SW-C(config)#spanning-tree backbonefast

Removing the SW-A to SW-B link..
(click on images to see in full size)

Now we can see SW-B is sending one Inferior BPDU only.. And then SW-C did not wait for the MaxAge time of 20s to go to Listening state..

UplinkFast for direct Root Port failure was a lightning fast feature. But BackboneFast can only reduce 50s of default convergence time to 30s by eliminating the MaxAge time.

When reverted back the time taken to re-converge was same (30s).

What happened at the back end?

What really happened was when the BackboneFast is enabled, SW-C sends a special message called RLQ (Root Link Query) to SW-A; the Root to find whether the Root is active or not. When SW-C got reply from SW-A, it assumes SW-B has lost the connection to the Root and immediately moves it's Blocking port to Listening > Learning > Forwarding states ignoring the 20s default MaxAge time.