Types of Spanning Tree Protocol (STP)

Types of Spanning Tree Protocol (STP)

This section reviews the three traditional types of Spanning Tree Protocol (STP) that are encountered in switched networks and how they relate to one another. No specific configuration commands are associated with the various types of STP here. Instead, you need a basic understanding of how they interoperate in a network.

Common Spanning Tree

The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches. It also specifies only a single instance of STP that encompasses all VLANs. This instance is referred to as the Common Spanning Tree (CST). All CST BPDUs are transmitted over trunk links using the native VLAN with untagged frames.

Having a single STP for many VLANs simplifies switch configuration and reduces switch CPU load during STP calculations. However, having only one STP instance can cause limitations, too. Redundant links between switches will be blocked with no capability for load balancing. Conditions also can occur that would cause CST to mistakenly enable forwarding on a link that does not carry a specific VLAN, whereas other links would be blocked.

Per-VLAN Spanning Tree

Cisco has a proprietary version of STP that offers more flexibility than the CST version. Per-VLAN Spanning Tree (PVST) operates a separate instance of STP for each individual 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. One link might forward one set of VLANs, while another redundant link might forward a different set.

Because of 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 occur. Each requires a different trunking method, so BPDUs are never exchanged between STP types.

Per-VLAN Spanning Tree Plus

Cisco has a second proprietary version of STP that allows devices to interoperate with both PVST and CST. Per-VLAN Spanning Tree Plus (PVST+) effectively supports three groups of STP operating in the same campus network:

■ Catalyst switches running PVST

■ Catalyst switches running PVST+

■ Switches running CST over 802.1Q

Table 2-14 summarizes the three STP types and their basic functions.

Type of STP	Function
CST	1 instance of STP, over the native VLAN; 802.1Q based
PVST	1 instance of STP per VLAN; Cisco ISL based
PVST+	Provides interoperability between CST and PVST; operates over both 802.1Q and ISL

To do 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 as untagged frames over the native VLAN. BPDUs from other instances of STP (other VLANs) 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 forward them on to downstream neighbors without interpreting them first. Eventually, the tunneled BPDUs reach other PVST+ switches where they are understood.