VTP Pruning
To preserve bandwidth, you can configure the VTP to reduce the number of broadcasts, multicasts,
and other unicast packets. This is called VTP pruning. VTP restricts broadcasts to only
trunk links that must have the information. If a trunk link does not need the broadcasts, the
information is not sent. VTP pruning is disabled by default on all switches.
Figure 14.8 shows that if a switch does not have any ports configured for VLAN 5 and a
broadcast is sent throughout VLAN 5, the broadcast would not traverse the trunk link going to
the switch without any VLAN 5 members.
Enabling pruning on a VTP server enables pruning for the entire domain, and by default,
VLANs 2 through 1005 are eligible for pruning. VLAN 1 can never prune.
Use the following command to set VLANs to be eligible for pruning:
Terry_4000> (enable) set vtp pruneeligible ?
Usage: set vtp [domain <name>] [mode <mode>]
[passwd <passwd>] [pruning <enable|disable>]
[v2 <enable|disable> (mode = client|server|transparent
Use passwd '0' to clear vtp password)
Usage: set vtp pruneeligible <vlans>
(vlans = 2..1000
An example of vlans is 2-10,1000)
Terry_4000> (enable) set vtp pruneeligible 2
Vlans 2-1000 eligible for pruning on this device.
VTP domain Globalnet modified.
476 Chapter 14 VLANs, Trunks, and VTP
FIGURE 1 4 . 8 VTP pruning
Notice once again that when you enable a VLAN for pruning, by default, it configures all the
VLANs. Use the following command to clear the unwanted VLANs:
Terry_4000> (enable) clear vtp pruneeligible 3-1005
Vlans 1,3-1005 will not be pruned on this device.
VTP domain Globalnet modified.
Terry_4000> (enable)
To verify the pruned state of a trunk port, use the show trunk command.
To set pruning on the 2950 and 3550, head into VLAN database mode. The command vtp
pruning enables the pruning process while the command switchport trunk pruning vlan
remove vlan-id removes VLANs from the list of pruning-eligible VLANs:
Terry_2950#vlan database
Terry_2950(vlan)#vtp ?
client Set the device to client mode.
domain Set the name of the VTP administrative domain.
password Set the password for the VTP administrative domain.
pruning Set the administrative domain to permit pruning.
server Set the device to server mode.
transparent Set the device to transparent mode.
v2-mode Set the administrative domain to V2 mode.
Terry_2950(vlan)#vtp pruning ?
v2-mode Set the administrative domain to V2 mode.
<cr>
Host B
VLAN 5 broadcast
traffic pruned here.
C500-4
C500-2
C500-3 C500-1
C500-5
C500-6
VLAN 5
Host A
VLAN 2 VLAN 4 VLAN 5
VTP pruning limits VLAN traffic to those links that support the VLAN.
802.1Q Tunneling 477
Terry_2950(vlan)#vtp pruning
Pruning switched ON
Terry_2950(vlan)#
Terry_2950#
Terry_2950#configure terminal
Terry_2950 (config)#interface fa 0/10
Terry_2950 (config-if)#switchport trunk pruning vlan remove 2-5,10
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