The classic "Cisco Lan Switching" chapter sixth (13): All of this Per vlan!

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It seems that the spanning tree protocol is still not perfect enough to be satisfying, in fact, because all of the discussions we discussed earlier in this chapter use only one VLAN. In other words, Cisco uses an STP instance in each VLAN , which is often referred to as Pvst: Spanning Tree per VLAN.

In other words, each VLAN can have a different root bridge and an active topology. For example, Figure 6-23, part A, is the active network topology diagram for VLAN 2, and parts B is the active network topology for VLAN3.Figure 6-23. PVST allows-to-Create Different Active topologies for each VLAN
In fact, that's why in the previous sectionSet SpantreeThe command has aVLANParameters. Each VLAN can have a different collection of active paths, and each VLAN can have a different clock value. Cisco has two important hints for using an instance on each VLAN:

• Allows you to use many excellent features, such as load balancing and per-VLAN traffic.
• will make you miserable (if you don't know what you're doing).

The goal now is to show you how to maximize 1th and avoid 2nd.
In general, a multi-instance spanning tree gives you an amazing tool to control your network. It is important to note that many service providers use only one STP instance for all VLANs, which not only gives you more control over the network, but also causes you to ruin the network. For example, suppose a single spanning tree instance in VLAN 1 determines the active topology of all VLANs, and if you remove VLAN 5 on the link used by VLAN 1, and that link is part of the active topology of all VLANs, then VLAN 5 is isolated, and the user of VLAN 5 is tragic.
The seventh chapter describes how to use multi-instance STP to accomplish advanced tasks such as STP load balancing.

The classic "Cisco Lan Switching" chapter sixth (13): All of this Per vlan!