VTP Pruning

JJBladester

I read the following from a cisco support documentation page (Catalyst 6500 Release 12.2SX Software Configuration Guide - VLAN Trunking Protocol (VTP)  [Cisco Catalyst 6500 Series Switches] - Cisco Systems)

VTP pruning enhances network bandwidth use by reducing unnecessary flooded traffic, such as broadcast, multicast, unknown, and flooded unicast packets. VTP pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the appropriate network devices.

I thought the whole point of VLANs was to shrink broadcast domains to just hosts on particular VLANs. Then why is pruning necessary to prevent broadcasts from going out over all switch ports? I thought VLAN tagging was used for that purpose.

pham0329

Switch A <---trunk---> Switch B -> Host A


VTP is used in this example, so all vlans on switch B are also on Switch A. Host A is in vlan 10, and generate a broadcast. Switch B receives it, and send it out the trunk to Switch A.

Switch A gets the frame....and does nothing with it because it has no trunks or any host in vlan 10. In this case, If VTP pruning were implemented, it would prevent Switch B from sending the frame to Switch A because Switch A doesn't need anything for Vlan 10.