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New Member

Vtp Pruning

Hi there

When pruning is enabled , no unknown unicasts,multicast and broadcasts are sent to switches which doesn't have ports assign to a certain vlan, eg vlan 3

If I assign a port to vlan 3 it will then start forwarding the traffic, but how does VTP get the information from the switch.

The only packets coming from a Client is a adv-request



Cisco Employee

Re: Vtp Pruning

As soon as a port in VLAN3 becomes 'connected', the switch knows that it has an active port in that VLAN and will send a VTP Join message over the trunks where that VLAN was initially pruned. That way, broadcasts, multicasts and unknown unicasts will be sent to the switch and the new user in VLAN3.

New Member

Re: Vtp Pruning


When you say connected, do you mean say, a pc is switched on and the port beomes active. So on a remote switch, even if all of the ports are in vlan 3, if none of the ports are active (no pc's switched on), this switch will be pruned until a pc is turned on.

Am I correct in thinking this.


New Member

Re: Vtp Pruning

The VTP Join Message seems to be shrouded in mystery., which has protocol decodes for VTP Summary Advertisements, Subset Advertisements, and Advertisement Requests, does not have the decode for VTP Join Messages nor does it include any information on the low level behavior of VTP Pruning. The Cisco Press book, Cisco LAN Switching, also is conveniently silent on this topic.

With VTP Pruning enabled, I observe one VTP Join message from switch every 6 seconds on 802.1Q trunks. Can anyone provide the VTP Join Message protocol decode (Network Associates Sniffer can't) and an explanation of why the periodicity is 6 seconds?

My understanding is that once a port is assigned to VLAN3 (in our example here), the switch informs the upstream neighbor. A host does not necessarily need to be connected to the port.