Introduction and overview of STP and RSTP

STP (Spanning Tree Protocol) is a layer two protocol. It allows us to add additional links in the network for backup. Backup links protect the network from going down if the primary links fail. However, they also create switching loops. Switching loops create broadcast storms and make the CAM table unstable. These issues block the network from properly functioning. STP addresses these issues. It logically blocks the links that create loops and monitors the network for a change. If a primary link goes down, it enables the backup link. Since it keeps only one link for each destination, no switching loop occurs in the network.

There are many variations of STP. Digital Equipment Corporation (DEC) created the original version of STP for its switches. Later, IEEE created an open standard version of STP. Both versions are not compatible with each other. You can use only one in the network. If you run both in the same network, the network will face switching loops.

Most modern networks either use IEEE's STP or their own variation of STP. Cisco uses its own variation. Cisco's STP is called PVST (Per VLAN Spanning-Tree). The main difference between STP and PVST is that STP runs only one instance on the switch, while PVST runs a separate instance for each VLAN. For example, if a switch has five VLANs, STP will run only one instance, while PVST runs five. Besides this, PVST also has some add-on features, such as PortFast and EtherChannel. PVST is a Cisco proprietary protocol. It works only on Cisco switches. It is, by default, enabled on Cisco switches.  

stp vs pvst

Since PVST uses the same framework and terminology STP uses, it's common to use the term STP for PVST. If you ever get confused about which variation the switch is running, you can check the manufacturer information of the switch. If it is a Cisco switch, it will be PVST. If it is a non-Cisco switch, it will be STP.

IEEE first standardized STP as part of the IEEE 802.1D standard back in 1990. At that time, convergency was not a matter of concern. Convergence is the time STP takes to react to a network change. For example, if a link goes down, convergency is the time STP takes to enable the backup link. STP takes fifty seconds to select and use the backup links. Most modern networks can't afford a fifty-second delay on every network change. If the network has a flapping link, this delay can bring the entire network down.

IEEE updated its STP to reduce the convergency time and meet modern network requirements. The updated version is called RSTP (Rapid Spanning Tree Protocol). Most modern networks use RSTP instead of STP. Cisco also updated its variation. It is called RPVST. RSTP/RPVST is the upgraded version of STP/PVST. It uses the same framework STP/PVST uses. To learn RSTP/RPVST, you must understand STP/PVST first. If you know STP/PVST, you already know most of RSTP/RPVST.

IEEE's STP vs Cisco PVST

CCNA Exam Objectives

CCNA exam objectives include the following topics related to STP and RSTP.

Interpret basic operations of Rapid PVST+ Spanning Tree Protocol

  • Root port, root bridge (primary/secondary), and other port names
  • Port states (forwarding/blocking)
  • PortFast

STP (PVST) and RSTP (RPVST) Tutorials

The following tutorials explain STP (PVST) and RSTP (RPVST) in detail. This tutorial series first presents the basic concepts and fundamentals of STP (PVST). Later, it describes the configuration and testing steps for STP (PVST) and RSTP (RPVST). Finally, it explains how RSTP (RPVST) differs from STP (PVST).

Conclusion

STP (PVST) and RSTP (RPVST) are spanning-tree protocols. They remove loops from the network. STP (PVST) is an old protocol. It takes a longer time than RSTP (RPVST) to reach convergence. RSTP (RPVST) is a modern protocol. It replaces STP (PVST). RPVST is the default protocol on all Cisco switches.

ComputerNetworkingNotes CCNA Study Guide Introduction and overview of STP and RSTP