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The command enables the BackboneFast feature, which should be enabled on all routers that contain an Ethernet switch network module. BackboneFast provides quicker convergence on the network backbone after a spanning-tree topology change by enabling the switch to detect an indirect link failure and to start the spanning-tree calculation faster than the normal spanning-tree rules would allow.


Cover of Cisco IOS in a Nutshell, 2nd Edition


Typically the Backbone Fast feature will need to be enabled on all the switches. What happens is that when the switch his a inferior BPDU on the root port then it will flush the CAM instead of waiting.