Cisco has developed several ways to make the standard Spanning Tree protocol more capable. These ways are; PortFast, UplinkFast, and BackboneFast.
Understanding Spanning Tree BackboneFast
We use the STP protocol to block loops that may occur in Layer 2 switches in a redundant network environment. The STP protocol also finds alternatives to link failures. As a result, with the Spanning Tree Protocol, we can continue our business without having too many delays on the network.
However, we face with some performance problems compared to the standard STP method. STP convergence lasts for 50 seconds by default. For example, it is absolutely unacceptable for a server farm that does very serious work to stay down for 50 seconds.
When configuring Switches in a Layer 2 network with redundant routes, we need to properly configure the Spanning Tree Protocol. For example, a client connected to the Switch can be configured as a PortFast port that the Switch is connected to by excluding it from the STP convergence process.
As another example, we can provide faster convergence by enabling the UplinkFast feature on a Switch with Blocking Port. As another example, we can enable the BackboneFast feature.
What is BackboneFast in STP?
Spanning Tree Protocol The BackboneFast feature is used to recover an indirect link failure. What is the indirect link error? If the cable between two Switches suddenly fails, it is an indirect link fault.
In the above image, an indirect link error will occur when the link between Switch0 and Switch1 is abruptly interrupted. Switch1 will determine a link fault for the directly connected cable and will assume itself the Root Bridge to not receive any BPDU from the Root Bridge.
Switch1 sends a BPDU packet to Switch2 telling it to be the new Root Bridge. Switch2, receiving BPDU package, realizes that this package is a new BPDU package and understands that this package is inferior when compared to the old BPDU. It will then cancel the new BPDU package.
As a result, Switch2 will start the max age timer (default 20 seconds) for the old BPDU packet. At the end of this period, Switch2 will receive the interface to Switch1 from Blocking mode to Listening mode and send a BPDU packet to Switch1. Switch1 will notice that Switch2 is not the Root Bridge and will not send BPDU packets again.
Switch2 will be in Forwarding mode by continuing from the Listening mode. And as a result of all this, a total of 50 seconds will pass.
Spanning Tree The BackboneFast feature will reduce latency from 50 seconds to 30 seconds. In the above image, the BackboneFast feature is enabled on all Switches to skip the max age timer. And therefore, the max age timer will not step in and we will save for 20 seconds.
What is Inferior Packet?
If any Switch on the network is receiving an inferior packet, it means that the adjacent Switch has lost its Root Bridge connection. When it can not reach a Switch to Root Bridge, it sends an inferior packet to its neighbor.
Enabling and Verifying the BackboneFast Feature
It is very easy to enable the BackboneFast feature on Cisco switches. To activate the BackboneFast feature in Global Config mode on the Cisco Switch, execute the following command.
Switch# conf t
Switch(config)# spanning-tree backbonefast
To verify the BackboneFast feature, you can verify it in Privileged mode by executing the following show command.
Switch# show spanning-tree backbonefast
How to Enable Spanning Tree BackboneFast on Cisco Switches ⇒ Video
You can watch how to enable Spanning Tree BackboneFast on Cisco Switches using GNS3 and you can better understand the BackboneFast feature. Also, subscribe to our YouTube channel to support us!
The PortFast, UplinkFast, and BackboneFast features used on Cisco switches are Cisco-specific and can not be used for other networking brands. After briefly mentioning STP Backbone Fast, we have observed the latency on the network without Backbone Fast.
If your network uses the standard STP protocol, it is recommended that you enable this feature. You can also use the Rapid Spanning Tree Protocol, which collects all the features of the standard STP protocol under a single protocol.