5.3 VLANs and STP
5.3.3 Per-VLAN Spanning Tree plus (PVST+)
As discussed earlier, 802.1Q has defined standards-based technologies for handling VLANs. To reduce the complexity of this standard, the 802.1 committee specified only a single instance of Spanning Tree for all VLANs. Not only does this provide a considerably less flexible approach than the PVST adopted by Cisco, it also creates an interoperability problem. To address both of these issues, Cisco introduced the Per-VLAN Spanning-Tree Plus Protocol (PVST+) in 4.1 code on the Catalyst 5000s (all Catalyst 4000s and 6000s support PVST+). This feature allows the two schemes to interoperate in a seamless and transparent manner in almost all topologies and configurations.

There are both advantages and disadvantages to using a single Spanning Tree. On the upside, it allows switches to be simpler in design and place a lighter load on the CPU. On the downside, a single Spanning Tree precludes load balancing and can lead to incomplete connectivity in certain VLANs (the single STP VLAN might select a link that is not included in other VLANs). Given these trade-offs, most network designers have concluded that the downside of having one Spanning Tree outweigh the benefits. Although the initial release of 802.1Q specified only a single instance of STP, the IEEE is currently working on multiple instances of STP in the 802.1S group.
PVST+ allows for three types of regions in the network:

  • A group of traditional (pre-4.1) Catalyst switches form a PVST region with each VLAN using a separate instance of STP.
  • Pure 802.1Q switches use a single instance of STP, the Mono Spanning Tree (MST). A group of these switches forms an MST region.
  • Catalyst Switches running 4.1 and later code form a PVST+ region.

Given that pure 802.1Q switches support 802.1Q-style trunks and PVST switches support only ISL trunks, these regions can be connected in only a limited set of combinations:

  • PVST and PVST+ regions can connect over ISL trunk links.
  • MST and PVST+ regions can connect over an 802.1Q trunk.

Notice that an MST and PVST region cannot be connected via a trunk link. Although it is possible to provide a nontrunk connection between the two regions by using an access (nontrunk) link, this is of limited use in real-world networks. The Figure illustrates the three types of STP regions and potential linkages.