Maintaining and Configuring STP
Comparing Common Spanning Tree and Per VLAN Spanning Tree – PVST

The 802.1D standard for spanning tree assumes that a single instance of spanning tree runs for all VLANs. This is known as Common Spanning Tree (CST). Per VLAN Spanning Tree (PVST) provides the option to potentially establish a root bridge on a VLAN by VLAN basis.

PVST is fully compatible with the 802.1Q trunking protocol and with ISL. PVST runs the same STA that 802.1D, and provides the same functionality, to prevent Layer 2 loops. The difference is that PVST is still a Cisco proprietary protocol and runs a separate instance of the STA for each VLAN. This means that for every VLAN created, a separate root bridge, a separate set of designated switches, and associated port roles and states are calculated.

Example: Comparing CST and PVST
In the example, the CST operation causes all the uplinks from the access layer to go to one of the distribution switches. This is undesirable because these uplinks are fully loaded with traffic while alternate path uplinks are inactive. Further should you wish to configure the default gateway for each VLAN to be on alternate distribution switches, then traffic will have to traverse the distribution to distribution link for half of the VLANs.

When PVST is used, each distribution switch is configured to be the primary root for each alternate VLAN. As access switches usually have a data and a voice VLAN, they can be configured to be on alternate distribution switches. Also, this way the dual uplinks from the access switches are both active, but for different VLANs.


Lab Activity

e-Lab Activity: Implementing Spanning Tree Protocol

In this lab, the student will configure basic spanning-tree and observe its default behavior.

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