5.5 Redundant Links
5.5.7 Configuring BackboneFast
BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from its designated bridge. An inferior BPDU identifies one switch as both the root bridge and the designated bridge.

When a switch receives an inferior BPDU, it indicates that a link to which the switch is not directly connected (an indirect link) has failed (that is, the designated bridge has lost its connection to the root bridge). Under normal Spanning-Tree rules, the switch ignores inferior BPDUs for the configured maximum aging time.

The switch tries to determine if it has an alternate path to the root bridge. If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root bridge. If the inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity to the root bridge, causes the maximum aging time on the root to expire, and becomes the root switch according to normal Spanning-Tree rules.

If the switch has alternate paths to the root bridge, the switch uses these alternate paths to transmit a new kind of BPDU called the root link query protocol data unit (PDU). The switch sends the root link query PDU out all alternate paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes the maximum aging time on the ports on which the switch received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes the maximum aging timers on the ports on which the switch received an inferior BPDU to expire. If one or more alternate paths can still connect to the root bridge, the switch makes all ports on which it received an inferior BPDU the designated ports and moves these ports out of the blocking state (if the ports were in the blocking state), through the listening and learning states, and into the forwarding state.

Use the following commands to configure and verify BackboneFast in the switch block. The switch software also provides commands that allow configuring the operation of BackboneFast. This feature enables faster convergence in case of a backbone link failure.

Enable BackboneFast on a set command-based switch by entering the following command:

Switch> (enable) set spantree backbonefast

Verify that BackboneFast is enabled by entering the following command:

Switch> (enable) show spantree backbonefast

Lab Activity
  In this lab activity, you will learn how to use PortFast, UplinkFast, and BackboneFast to improve the time it takes for the spanning-tree algorithm to configure the mode of the port.