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In normal STP 802.1D operation, if one of the leaf switches receive an inferior BPDU, port waits for max-age timer to expire and then it goes through listening and learning stage (50 seconds in total). Backbone fast feature, allows to skip max-age phase and go straight into listening and learning.

No Backbone Fast

S1 is a root bridge

Link between S2 and S3 gets damaged. S2 thinks is a root and starts sending BPDU towards S3.

S3 ignores inferior BPDU until max-age timer of eth0/1 expires (default 20 seconds) and once in listening state it forwards S1 BPDU towards S2. S2 learns that it is not a root bridge and connection is restored. This process took 50 seconds in total = max-age timer (20s) + forward delay timer (listening state 10s) + forward delay timer (forwarding state).

Backbone Fast

With Backbone Fast enabled, as soon as S3 receives inferior BPDU it sends RLQ out of all non-designated ports (apart from port that received inferior BPDU). If original root bridge is still live, S3 starts sending BPDU towards S2 without waiting for max-age timer. Entire process takes 30 seconds instead of 50.

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