Section 4.8: STP Types

There are three types of STP that are encountered in switched networks. These are: Common Spanning Tree (CST), Per-VLAN Spanning Tree (PVST), and Per-VLAN Spanning Tree Plus (PVST+). There are no specific configuration commands associated with the various types of STP. You should have a basic understanding of how the various types of STP interoperate in a network.

4.8.1: Common Spanning Tree (CST)

The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches. It specifies a single instance of STP for all VLANs. This is referred to as the Common Spanning Tree (CST) or the Mono Spanning Tree (MST). All BPDUs are transmitted over the management VLAN (VLAN1). Having a single STP for many VLANs simplifies switch configuration and reduces switch CPU load during STP calculations. However, the STP can cause limitations. Redundant links between switches will be blocked with no capability for load balancing. Conditions can also occur that would cause forwarding on a link that does not support all VLANs, while other links would be blocked.

4.8.2: Per-VLAN Spanning Tree (PVST)

Per-VLAN Spanning Tree (PVST) is a Cisco proprietary STP that offers more flexibility than CST. It operates a separate instance of STP for each VLAN. This allows the STP on each VLAN to be configured independently, offering better performance and tuning for specific conditions. Multiple Spanning Trees also make load balancing possible over redundant links when the links are assigned to different VLANs. Due to its proprietary nature, PVST requires the use of Cisco Inter-Switch Link (ISL) trunking encapsulation between switches. In networks where PVST and CST coexist, interoperability problems will occur as each requires a different trunking method. Therefore BPDUs will not be exchanged between PVST and CST.

4.8.3: Per-VLAN Spanning Tree Plus (PVST+)

Per-VLAN Spanning Tree Plus (PVST+) is another Cisco proprietary STP. It does, however, allow devices to interoperate with both PVST and CST. (PVST+) effectively supports three groups of STP operating in the same campus network: Catalysts running PVST; Catalysts running PVST+; and switches running CST/MST over 802.1Q. To accomplish this, PVST+ acts as a translator between groups of CST switches and groups of PVST switches. PVST+ can communicate directly with PVST by using ISL trunks. To communicate with CST, however, PVST+ exchanges BPDUs with CST on VLAN1. BPDUs from other instances of STP are propagated across the CST portions of the network by tunneling. PVST+ sends these BPDUs by using a unique multicast address so that the CST switches will forward them on to downstream neighbors. Eventually, the tunneled BPDUs will reach other PVST+ switches where they are understood.