VLAN Trunking Protocol (VTP) Tree Optimization
The initial configuration built by VLAN Trunking Protocol (VTP) should be reviewed and managed, or strategically pruned to optimize network traffic. VTP solves one problem for you — and immediately creates a new one.
Without VTP, you would need to manually create VLANs on all your switches or at least on switches that support devices or pass traffic for that VLAN.This allows traffic, especially broadcast traffic, for that VLAN to pass only through switches that are required in the communication process. The issue you encounter when using VTP is that the VLAN is propagated over trunk links to all of your network switches.
The result is that broadcast traffic, as shown in the following figure, will be sent to all switches on your network. This would not be a concern if you had network devices in that VLAN existing on all switches, but you do not; therefore, you have created extra network traffic for no reason.
In this case, there is a VLAN2 user on only two switches, interconnected by a third; you need to have the VLAN visible only on switches 1, 3, and 6.
Enter VTP pruning, a solution that eliminates the need to manually maintain the VLAN information on switches. The pruning feature prevents VLAN data from traversing needless links by monitoring traffic. You enable VLAN pruning on your switch in Global Configuration mode. After you enable VTP pruning, VLAN traffic will be evaluated for a very short time.
After VTP determines what switches require what VLAN traffic to send to the switches, it will filter the traffic. After the filtering process starts, much of the background traffic (such as broadcasts) on your network will be reduced, as shown in the following figure. This filtering is not a permanent state because when ports on the other switches are put in the VLAN, the required VLANs for those switches will be unpruned.
Only VLANs 2 to 1001 are pruned by default. VLAN1 is used for system tasks, such as carrying management data, so it will never be pruned. VTP pruning is not compatible with switches that are running in Transparent mode (because it is not able to manage them).