5.3 VLANs and STP
5.3.1 Per-VLAN STP
One of the biggest advantages of using a switch instead of a bridge is the ability to implement VLANs. However, the implementation of VLANs does impact the behavior of STP. Cisco and the IEEE 802.1Q committee approach the Spanning-Tree and VLAN issue in very different ways. Some of the major methods for reconciling STP and VLANs are listed as follows:
  • Common Spanning Tree (CST) is the IEEE 802.1Q solution to VLANs and Spanning Tree. CST defines a single instance of Spanning Tree for all VLANs. BPDU information runs on VLAN 1.
  • Per-VLAN Spanning Tree (PVST) is a Cisco proprietary implementation. PVST requires Cisco Inter-Switch Link (ISL) encapsulation in order to work. PVST runs a separate instance of STP for every VLAN.
  • PVST+ is a Cisco proprietary implementation that allows CST information to be passed correctly into PVST. A solution to the scaling and stability problems associated with large Spanning-Tree networks is to create separate instances of PVST.

Because each VLAN is a logical LAN segment, one instance of STP maintains a loop-free topology in each VLAN. Although many switches support a maximum of 1005 VLANs, you can enable Spanning-Tree Protocol on a maximum of only 64 VLANs at one time. If you configure more than 64 VLANs, you can still operate the other VLANs with STP disabled. By default, STP is enabled on VLANs 1 through 64.

Having a separate instance of Spanning Tree per VLAN as shown in Figure reduces the recovery time for STP recalculation, and hence increases the reliability of your network in the following ways:

  • Reduces overall size of Spanning-Tree topology
  • Improves scalability and decreases convergence time
  • Provides faster recovery and better reliability (see Figure )

Disadvantages of a Spanning Tree per VLAN include:

  • Utilization of switch to support Spanning-Tree maintenance for VLAN
  • Utilization of bandwidth on trunk links to support BPDUs for each VLAN