Vtp pruning question

SiRhOSEven

Hi guys, I'm really confused on VTP Pruning. We know that:

1. VTP pruning keeps away unnecessary broadcast from crossing the trunk links.
2. VLAN creates another broadcast domain

So why enable VTP pruning when VLANS are on a separate broadcast domain?

Please help. Thanks,

EdTheLad

You have 3 switches connected as follows:

SW1---SW2----SW3

SW2 is the vtp server, with 2 vlans configured, vlan 100 & 200
vtp will send these vlans to SW1 and SW3; the connected links are trunks.

All three switches have vlan 100 & 200; SW1 has 2 access ports in vlan 100; SW2 has 1 access port in vlan 100 and 1 IN VLAN 200, SW3 has one access port in vlan 200. A PC connected on SW1 access port sends an arp i.e. a L2 broadcast on vlan 100, this broadcast is received by SW1 and flooded to SW2 which again floods to SW3. SW3 does not have any access ports in vlan 100 so it cannot respond, why are we using bandwidth on the port for no reason? How can we stop the vlan 100 broadcast being sent to SW3? by removing vlan 100 off the trunk between SW2 and SW3. VTP pruning will allow the trunk to dynamically remove a vlan when there are no active ports in that vlan on the switch.