Configuring BackboneFast
What is BackboneFast?

BackboneFast addresses the situation where an indirect failure causes a topology change and therefore a switch must find an alternative path through an intermediate switch. BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from its designated bridge. Under normal spanning-tree rules, the switch ignores inferior BPDUs for the configured maximum aging time, as specified by the agingtime variable of the set spantree maxage command.

Example: BackboneFast Operation
BackboneFast operation is best illustrated by the failure of the link between the root and the backup root bridge. The backup root bridge does not assume that the root bridge is still available. The backup switch will immediately block all previously forwarding ports and then transmit configuration BPDUs claiming root responsibility.

When the access switch receives the BPDU of the backup root bridge, the access switch views the BPDU as inferior because its own root port is still active, and the last indication it has is that the backup root bridge is the designated root bridge. If configured for BackboneFast, the access switch then transmits a special root query message to explicitly determine if the root bridge is still active. Upon receipt of a response to the root query message, the access switch sends a BPDU using its known root bridge parameters to the backup root bridge and cycles the port connected to the backup root bridge through the listening and learning states.

This differs from standard 802.1D spanning tree operation in that normally the blocked port does not process the received BPDUs until the maxage interval has expired. By using the BackboneFast feature, the network recovers from an indirect failure in two times the forward delay time, which is 30 seconds by default, rather than max_age plus two times forward delay time, which is 50 seconds by default.