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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 10 Configuring VTP
Understanding How VTP Version 1 and Version 2 Work
• Consistency Checks—In VTP version 2, VLAN consistency checks (such as VLAN names and
values) are performed only when you enter new information through the CLI or SNMP. Consistency
checks are not performed when new information is obtained from a VTP message, or when
information is read from NVRAM. If the digest on a received VTP message is correct, its
information is accepted without consistency checks.
Understanding VTP Pruning
Note Enabling VTP pruning on a VTP version 3 switch enables pruning only on the switch that you enable it
on. VTP pruning is not propagated as it is with VTP version 1 and VTP version 2.
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. By default, VTP pruning is disabled.
Make sure that all devices in the management domain support VTP pruning before enabling it. VTP
pruning is supported in supervisor engine software release 5.1(1) and later releases.
Note If you use routers to route between emulated LANs, you should disable VTP pruning in the VTP
management domain that contains the switches with ATM LANE modules installed (VTP pruning
messages are sent over the ATM LANE module because it is a trunk). You can also disable pruning for
the LANE VLANs by using the clear vtp pruneeligible command on all switches with ATM LANE
modules.
Figure 10-1 shows a switched network without VTP pruning enabled. Port 1 on Switch 1 and port 2 on
Switch 4 are assigned to the Red VLAN. A broadcast is sent from the host that is connected to Switch 1.
Switch 1 floods the broadcast and every switch in the network receives it even though Switches 3, 5,
and 6 have no ports in the Red VLAN.
Figure 10-1 Flooding Traffic without VTP Pruning
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