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| 3.3 |  | STP Enhancements |
| 3.3.4 |  | BackboneFast |

BackboneFast is a Catalyst feature that 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 means 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 STP rules, the switch ignores inferior BPDUs for the configured Max Age (the default is 20 seconds).

The role of BackboneFast is essentially to cheat this 20-second delay. When the switch receives the inferior BPDU, the switch tries to determine if it has an alternative path to the Root Bridge.

There are two cases to consider:

- First, if the inferior BPDU arrives on a blocked port, the Root Port and other blocked ports on the switch become alternative paths to the Root Bridge.
- Second, if the inferior BPDU arrives on the Root Port, all blocked ports become potential alternative 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 causing the Max Age on the root to expire, and becomes the Root Bridge according to normal STP rules. If the switch has alternative paths to the Root Bridge it uses these alternative paths to transmit a new kind of Protocol Data Unit (PDU) called the Root Link Query PDU. The switch sends the Root Link Query PDU on all potential alternative paths to the Root Bridge.

If the switch determines that it still has an alternative path to the root, it causes the Max Age on the ports on which it received the inferior BPDU to expire. The switch then makes all ports on which it received an inferior BPDU its Designated Ports and moves them out of the Blocking state (if they were in the Blocking state), through the Listening and Learning states, and into the Forwarding state. On the other hand, if the switch learns via the Root Link Query process that all the alternative paths to the Root Bridge have lost connectivity to the root, the switch causes the Max Age on the ports on which it received inferior BPDUs to expire, and a new STP topology is calculated. To illustrate this process, Figure shows a sample topology with no link failures. Switch A, the Root Bridge, connects directly to Switch B over link L1 and to Switch Cover link L2. The interface on Switch C that connects directly to Switch B is in the Blocking state.

If link L1 fails, Switch C cannot directly detect this failure, because it is not directly connected to link L1. Switch B is directly connected to the Root Bridge over L1, detects the failure, elects itself as the root and begins sending BPDUs to Switch C. When Switch C receives the inferior BPDUs from Switch B, Switch C assumes that an indirect failure has occurred. At that point, BackboneFast allows the blocked port on Switch C to move immediately to the Listening state without waiting for Max Age on the port to expire. BackboneFast then transitions the interface on Switch C to the Forwarding state, providing a path from Switch B to Switch A. This switchover takes approximately 30 seconds, twice the Forward Delay time if the default Forward Delay time of 15 seconds is set. This saves up to 20 seconds. Figure shows how BackboneFast reconfigures the topology to account for the failure of link L1.