Cisco Systems 4500 Understanding VTP Pruning

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 13 Configuring VLANs, VTP, and VMPS VLAN Trunking Protocol
database information. The updates sent out are honored by all the devices in the system. A VTP
secondary server can only back up to its NVRAM the VTP configuration received by using updates
from the VTP primary server.
The status of primary and secondary servers is a runtime status and is not a configurable option. By
default, all devices are initiated as secondary servers. Primary server status is needed only when
database updates are needed, and is obtained when the administrator issues a takeover message in
the domain. See the “Starting a Takeover” section on page 13-19.
Primary server status is lost upon reload of the device, or when switchover or domain parameters
change. Secondary servers back up the configuration and continue to propagate it. Because of that,
you may have a working VTP domain without any primary servers.
Understanding VTP Pruning
VTP pruning enhances network bandwidth use by reducing unnecessary flooded traffic, such as
broadcast, multicast, and 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.
By default, VTP pruning is disabled.
For VTP pruning to be effective, all devices in the management domain must either support VTP pruning
or, on devices that do not support VTP pruning, you must manually configure the VLANs allowed on
trunks.
Figure 13-2 shows a switched network without VTP pruning enabled. Interface 1 on Switch 1 and
Interface 2 on Switch 4 are assigned to the Red VLAN. A broadcast is sent from the host connected to
Switch 1. Switch 1 floods the broadcast and every network device in the network receives it, even though
Switches 3, 5, and 6 have no interfaces in the Red VLAN.
You can enable pruning globally on the Catalyst 4500 series switch (see the “Enabling VTP Pruning”
section on page 13-15).
Figure 13-2 Flooding Traffic without VTP Pruning
Figure 13-3 shows the same switched network with VTP pruning enabled. The broadcast traffic from
Switch 1 is not forwarded to Switches 3, 5, and 6 because traffic for the Red VLAN has been pruned on
the links indicated (Interface 5 on Switch 2 and Interface 4 on Switch 4).
Catalyst series
switch 4
Catalyst series
switch 5
Catalyst series
switch 3
Catalyst series
switch 6 Catalyst series switch 1
Catalyst series
switch 2
Interface 1
Interface 2
Red
VLAN
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