Backbone Fast

Sections:

Overview:

BackboneFast helps eliminate the need for the full STP convergence recalculation time in an event of a failure within the network backbone (the core part of the network)
BackboneFast helps speeds up the process of recovering from a root switch failure or a root port failure by quickly rerouting traffic
BackboneFast is built into the standard by default in RSTP/RPVST+ or MST

How it Works:

BackboneFast works by allowing a switch to detect when there is a suspected problem with the root switch or its root port
Switches that lose connectivity to the root bridge immediately send out TCN (Topology Change Notification) messages to quickly update their topology information
With BackboneFast enabled globally, the switch that detects the failure sends a special backbone query to neighboring switches asking whether they have heard about the failure or can see the root bridge
If a neighboring switch has knowledge of the failure, it will inform the querying switch, which can then begin the process of quickly finding a new path or reevaluating its role on the network
If a backbone query reveals that the failure has propagated, the affected switches can rapidly transition to a new topology instead of waiting for the full STP recalculation process
Triggered in cases where indirect link failures at the core (root switch) occur

When to use:

Commonly utilized on distribution or core layer switches where the failure of the root switch or root port has a significant impact on the overall network
Beneficial in larger networks or redundant core networks where multiple potential paths can be used to reach the root bridge
BackboneFast is manually configured in older versions of STP (802.1D STP/PVST)
Note: BackboneFast must be enabled on all switches in the domain if implementing in older versions of STP

Scenario Example

Switch (C) stops receiving BPDUs from Root switch (A)
Switch (C) sends out TCN messages towards switch (B) claiming to be the new root switch to re-converge the STP topology
With Switch (B) receiving superior BPDUs from switch (A), switch (B) also receives inferior BPDUs from switch (C)
Switch (B) queries the Root switch (A) to confirm it is still the root switch
If confirmed, Switch (B) discards the inferior BPDU from switch (C)
Switch (B) will then transition its blocking port towards switch (C) to forwarding to adjust the STP topology due to the indirect link failure

Spanning Tree