PVST – Per–Vlan Spanning Tree
– CISCO Owner Protocol
– Based on the STP protocol
– A Vlan – A Spanning Tree Instance
– Enabled by default on CISCO equipment
The PVST protocol is identical to the STP protocol except that it creates its topology by vlan.
The purpose of this article is to understand how pvST works on our architectures and how to properly configure it.
Difference between PVST and PVST
– Compatible with the 802.1q
– CST-compatible (Common Spanning Tree)
Choice of Root-bridge
By default, everyone has the same priority.
The switch with the lowest MAC address will therefore be voted "Root-Bridge".
In most cases, the oldest switch in the network has the lowest MAC address.
You will let the oldest switch in your network take responsibility for your network architecture?? Of course not!!
Let's take the following architecture as an example:
We assume that all our connections have an equal cost and that the network administrator has not touched anything at the Spanning-tree level. So we're in PVST mode. A protocol based on STP.
THE MAC addresses are:
– A – 0000.0000.0001
– B – 0000.0000.0002
– C – 0000.0000.0003
– D – 0000.0000.0004
– E – 0000.0000.0005
– F – 0000.0000.0006
– G – 0000.0000.0007
– H – 0000,000.0000
On this architecture, the Switch H has the lowest MAC address. So it becomes "Root-Bridge." The active links are as follows.
We agree, the most logical would be to put router B in "Root-Bridge". What gives us the following architecture:
To do this, we have two methods:
Tell our Switch to become "Root-Bridge"
Switch-B (config) spanning-tree vlan 10 root
The Switch B will look at the Bridge-ID of the current Root-Bridge and will change its priority to have a lower Bridge-ID.
Changing the priority value of switch B
Switch-B (config) spanning-tree vlan 10 priority XXXX
As you will have understood, this command requires the administrator to know the value of the Bridge-ID of the current Root-Bridge.
Since we use PVST (PER-VLAN Spanning Tree). We are obliged to do one of his two manipulations for all our vlans.
The Bridge-ID is made up of two elements:
– Mac Address
The Priority is also made up of two elements:
By default, the value is 32768 (always multiple of 4096).
You have to add up your two values to have the Bridge-ID's priority value
For the Vlan 10 , the bridge-ID's priority value will be by default of 32768-10 – 32778
How PVST works
Let's go back to the previous architecture with Vlans:
The Switch B was elected Root Bridge for the Vlans 10 and 20. The logical architecture becomes:
PVST assumes that only active equipment with the VLAN in question is included in the STP architecture.
If we want to force our PVST logic architecture, we can also intervene on the cost of the link with the following command:
Switch-B (config) - FastEthernet interface 0/1 Switch-B (config-if) spanning-tree vlan 10 cost X
Switch-B- spanning-tree show
Hoping this article has been helpful to you! Don't hesitate to let me know!!