What is BackboneFast in STP | How Does It Work?

Cisco has built several ways to boost the speed of the standard Spanning Tree rules. These ways include PortFast, which gives fast endport link, UplinkFast, which gives quick switch on spare lines, and BackboneFast.

STP BackboneFast Definition

Understanding BackboneFast in STP

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 some performance problems compared to the standard STP method. STP convergence (convergence/integration) takes 50 seconds by default as a result of a cable error occurring in the STP network. For example, a Server farm that does severe work is absolutely unacceptable to remain down for 50 seconds.

When configuring Switches in a Layer 2 network with redundant routes, we need to configure the Spanning Tree Protocol properly. 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.

Another example is that by enabling UplinkFast on Switch with Blocking Port, we can achieve faster convergence.

This feature is also used to recover an indirect link error. What is an indirect link error? If we answer the question, the sudden failure of the cable between the two Switches is an indirect link error.

How Does Backbonefast Work?

In the below image, an indirect link error will occur when the link between Switch0 and Switch1 is abruptly interrupted. Switch 1 will determine a link fault for the directly connected cable and will assume the Root Bridge does not receive any BPDU from the Root Bridge.

A Network Topology with Three Switches in Cisco Packet Tracer

Switch1 sends a BPDU packet to Switch2, telling them that it is the new Root Bridge. Switch2, receiving the 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.

The STP BackboneFast feature will reduce the delay here from 50 seconds to 30 seconds. By activating this feature on all Switches in the above image, the max-age timer duration is skipped. So, the max-age timer will not be activated, and we will save 20 seconds from this process.

What is an 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 the Root Bridge, it sends an inferior packet to its neighbor.

Enabling and Verifying the BackboneFast Feature

Execute the following command in Global Config mode to enable this protocol on Cisco Switches.

Switch# conf t
Switch(config)# spanning-tree backbonefast

To verify the operation, you can use the show command below in Privileged mode.

Switch# show spanning-tree backbonefast

Video

You can watch the video below to activate Backbone-Fast on Switches by using the GNS3 program, and you can also subscribe to our YouTube channel!

Watch on YouTube

FAQ About BackboneFast

Is the 50-second convergence time in standard STP really that terrible?

In a server farm, 50 seconds is like an eternity. Users lose their sessions. Operations are cut off halfway.

During this time, all layer 2 communication freezes. The Root Bridge election is redone. Blocked ports go into listening mode.

In short, this delay is unacceptable in finance and health systems. Standard STP’s max age and forward delay combine. As a result, you lose money per minute.

How do I tell an indirect link failure from a direct failure?

A direct failure happens on the cable connected to you. The switch immediately sees the line as down. The port light goes off.

An indirect failure, on the other hand, is on your neighbor’s line to the root. Your link is healthy but BPDU packets suddenly stop. The switch starts to think it is the root itself.

Right here, inferior BPDU comes into play. The neighbor device compares the old and new info. Then it starts the max age counter.

What does an inferior BPDU packet mean and what does it cause on the network?

When a switch loses its path to the root, it announces this. In the BPDU it sends, it introduces itself as the new root. This packet is called an inferior, that is, inadequate BPDU.

The neighbor switch keeps the current root info. It compares the priority of the new BPDU. It sees that it is lower and drops the packet.

But right after, it starts the max age timer. It waits for the old info to stay valid for a full 20 seconds. This waiting time gets long without backbone improvement.

Exactly which seconds does this backbone improvement feature save me?

In the standard process, the max age counter is 20 seconds. The port stays blocked until this counter is up. BackboneFast completely skips this counter.

This way the switch that gets the inferior BPDU reacts right away. The root query process starts within seconds. The blocked port immediately goes into listening mode.

As a result, the total time drops from 50 seconds to 30 seconds. Your network gets back up 20 seconds earlier. This gain saves the day in large topologies.

What should I do if this Cisco feature does not work in a mixed switch setup?

Different brand devices do not recognize BackboneFast. Only Cisco supports this shortcut. In this case, I suggest you switch to Rapid Spanning Tree.

RSTP standardizes all these improvements. It gathers PortFast, UplinkFast, and backbone acceleration under one roof. What is more, it is fully compatible with other vendors.

Because RSTP uses alternate port and backup port concepts. Convergence time drops to a few seconds. A proper design is enough instead of complex rules.

How do I enable this fast backbone feature from the command line and verify it?

A single command is enough in global configuration mode. Type ‘spanning-tree backbonefast’ and press enter. Repeat this process on all switches.

I use the ‘show spanning-tree backbonefast’ command for verification. In the output, you see that the feature is enabled. It also lists the statistics.

Do not forget, this command only turns on backbonefast. Separate configuration is needed for PortFast and UplinkFast. Check the command history to make sure you did not make a mistake.

Conclusion

The PortFast, UplinkFast, and BackboneFast features used on Cisco Switches are specific to Cisco and cannot be used for other network devices.

If you are using the standard STP protocol on your network, enabling this feature is recommended. You can also use the Rapid Spanning Tree Protocol, which gathers all the features covered by the standard one under a single protocol.

What is BackboneFast in Spanning Tree Protocol?