Cisco CCNA Certification Exam Tutorial:

VTP Configuration Revision Numbers and VTP Pruning

By Chris Bryant, CCIE #12933

Most VLAN Trunking Protocol (VTP) deployments are going to have two or more VTP servers, so when one VTP server sends a summary advertisement, how does the receiving VTP server know if that ad has the latest and greatest information?

Every VTP summary advertisement has a configuration revision number that is incremented by one when it updates its own VTP database.  That same number is placed into the outgoing VTP summary advertisement.  If the receiving switch's own VTP configuration revision number is lower than that of the incoming advertisement, the incoming ad's information is considered to be more recent and is accepted.

If the incoming ad's revision number is lower than that of the receiving switch, the incoming advertisement is considered out-of-date and is therefore ignored.

A Look At VTP Pruning

Consider this situation:  two switches are trunking, and each has ports in ten VLANs.  Of all those VLANs, the switches only have two in common.

The switches both have ports in VLANs 10 and 11, but have no other common VLANs.  By default, broadcast and multicast traffic destined for any VLAN will cross the trunk, resulting in a lot of unnecessary traffic crossing the link.

This default behavior can be stopped by enabling VTP pruning.  With VTP pruning enabled on these switches, a VLAN’s broadcasts will be sent across the trunk only when there are ports belonging to that particular VLAN on the opposite switch.  Broadcasts for VLANs 10 and 11 will go across the trunk, but not for the other VLANs.

You would think that VTP pruning is on by default, but it's not.  To turn it on, run vtp pruning and verify with show vtp status.

SW1(config)#vtp pruning
Pruning switched on

SW1#show vtp status
VTP Version                     : 2
Configuration Revision          : 3
Maximum VLANs supported locally : 64
Number of existing VLANs        : 7
VTP Operating Mode              : Server
VTP Domain Name                 : CCNA
VTP Pruning Mode                : Enabled

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com