Per-VLAN Spanning Tree (PVST)

Per-VLAN Spanning Tree (PVST)
The Spanning Tree Protocol does not scale well with large switched networks. In such networks,
delays can occur in receiving BPDUs. These delays can cause instability in the STP database and
convergence delay problems, which means that the network will not be forwarding frames.
To solve problems associated with late BPDUs and convergence delays, Cisco created a concept
entitled
Per-VLAN Spanning Tree (PVST)
. This creates smaller STP implementations,
which are easier for the switches to manage. Each VLAN has a unique Spanning Tree Protocol
topology for its root, port cost, path cost, and priority.
When running PVST, you still provide a loop-free network, but it is based within each
VLAN. Each switch has a spanning tree process running for each VLAN. If a switch has five
VLANs that it knows about, then it will have five instances of spanning tree running. The benefits
of having a PVST are as follows:

The spanning tree topology is smaller because all links will not necessarily support all VLANs.

The STP recalculation time is reduced when the switched network is converging.

The switched network is easier to scale.

Recovery is faster than with a large network that has one STP instance.

Administrative control of forwarding paths is permitted on a subnet basis.

Load balancing over redundant links is permitted when VLAN priorities are established for
those links.
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Chapter 16 
Using Spanning Tree with VLANs
There are, however, some disadvantages of using a spanning-tree-per-instance implementation:

The utilization on the switch is a factor because it needs to manage all the STP instances.

You must take into consideration that the trunk links have to support all the VLAN STP
information as well.

It requires ISL.

PVST is a Cisco proprietary protocol.