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| 3.5 |  | Evolution of Spanning Tree |
| 3.5.1 |  | PVST + |

PVST (Per-VLAN Spanning Tree), PVST+, and Mono Spanning Tree Modes
Common spanning tree (CST) is specified in the IEEE 802.1Q standard. CST defines a single instance of Spanning Tree for all VLANs. BPDUs are transmitted over VLAN 1. Per-VLAN spanning tree (PVST) is a Cisco-proprietary implementation requiring ISL trunk encapsulation. PVST runs a separate instance of STP for each VLAN. Among other things, this gives the ability to configure distinct Root Switches for each VLAN and configure Layer 2 load balancing.

Having a separate instance of spanning tree for each VLAN reduces the recovery time for STP recalculation and hence increases network reliability in the following ways:

- Reduces the overall size of the spanning tree topology
- Improves scalability and decreases convergence time
- Provides faster recovery and better reliability

Disadvantages of a spanning tree for each VLAN include the following:

- Utilization of switches (such as CPU load) to support spanning tree maintenance for multiple VLANs
- Utilization of bandwidth on trunk links to support BPDUs for each VLAN PVST+ is a Cisco-proprietary STP mode that allows CST and PVST to exist on the same network.

PVST+ provides support for 802.1Q trunks and the mapping of multiple spanning trees to the single spanning tree of non-Cisco 802.1Q switches. PVST+ is automatically enabled on Catalyst 802.1Q trunks. It runs one instance of STP per VLAN when Catalyst switches are connected by 802.1Q trunks. PVST+ is the default Spanning-Tree Protocol used on all Ethernet, Fast Ethernet, and Gigabit Ethernet port-based VLANs on Catalyst 4000 and 6000 family switches. PVST+ runs on each VLAN on the switch, ensuring that each has a loop-free path through the network.

PVST+ provides Layer 2 load balancing for the VLAN on which it runs. Different logical topologies can be crated using the VLANs on a network to ensure that all links will be used but no single link will be oversubscribed.

Each instance of spanning tree has a single Root Switch. This Root Switch propagates the spanning tree information associated with that VLAN to all other switches in the network. Because each switch has the same knowledge about the network, this process ensures that the network topology is maintained.

The PVST+ architecture distinguishes three types of zones or regions:

- A PVST zone or region
- A PVST+ zone or region
- A mono spanning tree zone or region

Mono spanning tree (MST) is the spanning tree implementation used by non-Cisco 802.1Q switches. One instance of STP is responsible for all VLAN traffic. Each zone or region consists of a homogenous type of switch. Connecting two ISL ports can connect a PVST region to a PVST+ region. Similarly, connecting two 802.1Q ports can connect a PVST+ region to an MST region. Notice that an MST and PVST region cannot be connected via a trunk link. Although it is possible to provide a non-trunk connection between the two regions by using an access link, this is of limited use in real world networks. The three types of STP regions and how they are linked are illustrated in Figure .

At the boundary between a PVST region and a PVST+ region, the mapping of spanning trees is one-to-one. At the boundary between an MST region and a PVST+ region, the spanning tree in the MST region maps to one PVST in the PVST+ region. The one it maps to is the CST. The default CST is VLAN 1.

All PVSTs, except the CST, are tunneled through the MST region. Tunneling means that BPDUs are flooded through the MST region along the single spanning tree that is present in the MST region. Do not confuse this use of the term tunnel with the point-to-point tunnels configured on Cisco routers and PIX firewalls. In general, tunnel refers to a mechanism that enables the propagation of packets or frames through a series of devices in such a way that the contents of the packets or frames are not changed in transit.