What is a Spanning Tree Protocol?

A Spanning Tree Protocol (STP) is a network protocol that prevents loops in a network topology by creating a logical tree-like structure of network links. This protocol is crucial because loops can cause broadcast storms, which can result in network congestion and ultimately, network failure.

There are several variations of the STP protocol, including the original STP, Rapid Spanning Tree Protocol (RSTP), and Multiple Spanning Tree Protocol (MSTP). These protocols differ in terms of their speed, complexity, and features.

Configuring a Spanning Tree with Cisco Commands

In this section, we will explain how to configure the STP protocol using Cisco commands. Specifically, we will use the example of configuring the RSTP protocol on a Cisco switch.

Enable RSTP on the switch
To enable RSTP on a Cisco switch, use the following command:

Switch(config)# spanning-tree mode rapid-pvst

This command enables the RSTP protocol on the switch and configures it to use the rapid-per-VLAN spanning tree (PVST) mode.

Configure the switch priority
Each switch in the network has a priority value, which determines the root bridge of the spanning tree. By default, the priority value is 32768. However, you can change this value using the following command:

Switch(config)# spanning-tree vlan  priority 

This command sets the priority value for a specific VLAN. For example, if you want to set the priority value for VLAN 10 to 16384, you would use the following command:

Switch(config)# spanning-tree vlan 10 priority 16384

Configure the root bridge
The root bridge is the switch that serves as the central point of the spanning tree. To configure a switch as the root bridge, use the following command:

Switch(config)# spanning-tree vlan  root primary

This command sets the switch as the root bridge for a specific VLAN. For example, if you want to set Switch 1 as the root bridge for VLAN 10, you would use the following command:

Switch(config)# spanning-tree vlan 10 root primary

Verify the spanning tree configuration
To verify the spanning tree configuration, use the following command:

Switch# show spanning-tree

This command displays information about the spanning tree, including the root bridge, port roles, and port states.

Conclusion

The Spanning Tree Protocol is an essential protocol for preventing network loops and ensuring network stability. In this article, we explained what the STP protocol is and how to configure it using Cisco commands. By following the steps outlined above, you can configure the RSTP protocol on a Cisco switch and ensure that your network is protected from loops and broadcast storms.