PVST (Per VLAN Spanning-Tree)

Since you're reading this, I assume you're familiar with how "classic" Spanning-Tree Protocol (STP) operates. If you're not, I recommend checking out my Introduction to Spanning-Tree Protocol (STP) tutorial before proceeding.

With that in mind, let's begin with a helpful diagram:

VLAN 10 is set up on SW1 and SW2, while VLAN 20 is configured on SW1, SW2 and SW3.

So, do we have a loop in VLAN 10? No, we don't, since it only spans the link between SW1 and SW2; however, there is a loop in VLAN 20 due to the connections involving SW1, SW2, and SW3.

How does STP address this issue? It's pretty straightforward: a separate STP is calculated for each VLAN. The oldest version of Spanning-Tree is known as CST (Common Spanning-Tree), defined in the 802.1D standard, which computes a single Spanning-Tree for all VLANs.

In contrast, PVST (Per VLAN Spanning-Tree) allows for a unique topology to be calculated for each VLAN. This is the default configuration on Cisco switches.

‼ Beginning from Cisco IOS 15.2(4)E Software Release, the STP default mode is Rapid PVST+.

Let's look at an example of three switches and two VLANs. Both VLANs are available on all connecting switches:

As illustrated above, with PVST, we can establish distinct root bridges for each VLAN. For instance, SW1 could serve as the root bridge for VLAN 10, while SW2 might be the root bridge for VLAN 20.

？But. Why would we want to configure it this way?

Here's an example to illustrate the benefits of why that configuration is a good idea:

If we designate a single switch as the root bridge for both VLANs, one of its interfaces will be blocked for both VLANs. In the diagram above, if SW1 is the root bridge for both VLAN 10 and VLAN 20, the Fa0/16 interface on SW3 will be blocked for both. This means no traffic can be forwarded through the Fa0/16 interface at all.

Now, consider that these might be 10 Gigabit interfaces - it would be unfortunate for such costly interfaces to remain unused!

If we select a different switch as the root bridge for VLAN 20, the results will change significantly:

In my example, by designating SW2 as the root bridge for VLAN 20, we can observe different results. Now, the Fa0/16 interface on SW2 is blocked for VLAN 10, while the Fa0/17 interface on SW1 is blocked for VLAN 20. This setup allows for better load sharing and balancing.

I hope you found this explanation insightful. If you found this lesson helpful, please drop us a comment below!

Happy PVST'ing!

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Thanks for reading: PVST (Per VLAN Spanning-Tree), Sorry, my English is bad:)