17-10
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 17 Configuring VLANs, VTP, and VMPS
VLAN Trunking Protocol
VTP Version 3
VTP version 3 supports the following features not supported in version 1 or version 2:
• Hidden password support—Supports the option of configuring the password as hidden or secret.
When the hidden keyword is specified, that password must be reentered if a takeover command is
issued in the domain. The secret key generated from the password string is saved in the
const_nvram:vlan.dat file. When configured with this option, the password does not appear in plain
text in the configuration. Instead, the secret key associated with the password is saved in
hexadecimal format in the running configuration. If the hidden keyword is not specified, the
password is saved in clear text in the const_nvram:vlan.dat file as in VTP version 1 and VTP
version 2.
When the secret keyword is specified, the password secret key can be directly configured.
• Extended VLAN database propagation support—In VTP version 2, VLAN configuration
information is propagated only for VLANs numbered 1 to 1000. In VTP version 3, information also
is propagated for extended-range VLANs (VLANs numbered 1006 to 4094).
• On Catalyst 4500 series switches running VTP version 1, VTP version 2, or VTP version 3, default
VLANs 1 and 1002 to 1005 cannot be modified.
Note VTP pruning continues to apply only to VLANs numbered 1 to 1000.
• Propagation of any database in a domain—In addition to propagating VLAN database information,
VTP can propagate Multiple Spanning Tree (MST) protocol database information.
• Disabling VTP—When VTP is disabled on a trunking port, it applies to all VTP instances on that
port. When VTP is disabled globally, the setting applies to all the trunking ports in the system.
• In VTP version 1 and VTP version 2, the role of a VTP server is to back up the database to NVRAM
and to allow the administrator to change database information. VTP version 3 introduces the roles
of VTP primary server and VTP secondary server. A VTP primary server is used to update the
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 17-18.
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.
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