Spanning Tree Concepts

Purpose of Spanning Tree

Broadcast Storm

A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available for legitimate traffic and the network becomes unavailable for data communication. This is an effective denial of service.

A broadcast storm is inevitable on a looped network. As more devices send broadcasts over the network, more traffic is caught within the loop, consuming resources. This eventually creates a broadcast storm that causes the network to fail.

There are other consequences of broadcast storms. Because broadcast traffic is forwarded out every port on a switch, all connected devices have to process all broadcast traffic that is being flooded endlessly around the looped network. This can cause the end device to malfunction because of the high processing requirements for sustaining such a high traffic load on the NIC.

Click the Play button in the figure to view an animation of a broadcast storm. When the animation pauses, read the text to the right of the topology. The animation will continue after the short pause.

In the animation:

1. PC1 sends a broadcast frame out onto the looped network.

2. The broadcast frame loops between all the interconnected switches on the network.

3. PC4 also sends a broadcast frame out on to the looped network.

4. The PC4 broadcast frame also gets caught in the loop between all the interconnected switches, just like the PC1 broadcast frame.

5. As more devices send broadcasts over the network, more traffic is caught within the loop, consuming resources. This eventually creates a broadcast storm that causes the network to fail.

6. When the network is fully saturated with broadcast traffic that is looping between the switches, new traffic is discarded by the switch because it is unable to process it.

Because devices connected to a network are regularly sending out broadcast frames, such as ARP requests, a broadcast storm can develop in seconds. As a result, when a loop is created, the switched network is quickly brought down.