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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 9 Configuring VTP
Understanding How VTP Version 1 and Version 2 Work
•
Transparent—VTP transparent switches do not participate in VTP. A VTP transparent switch does
not advertise its VLAN configuration and does not synchronize its VLAN configuration based on
received advertisements. However, in VTP version 2, transparent switches do forward VTP
advertisements that they receive out their trunk ports.
•
Off—In the three modes described above, VTP advertisements are received and transmitted as soon
as the switch enters the management domain state. In the VTP off mode, switches behave the same
as in VTP transparent mode with the exception that VTP advertisements are not forwarded.
Understanding VTP Advertisements
Each switch in the VTP domain sends periodic advertisements out each trunk port to a reserved multicast
address. VTP advertisements are received by neighboring switches, which update their VTP and VLAN
configurations as necessary.
The following global configuration information is distributed in VTP advertisements:
•
VLAN IDs (ISL and 802.1Q)
•
VTP domain name
•
VTP configuration revision number
•
VLAN configuration, including the maximum transmission unit (MTU) size for each VLAN
•
Frame format
Understanding VTP Version 2
If you use VTP in your network, you must decide whether to use VTP version 1, version 2, or version 3
(for details on version 3, see the
“Understanding How VTP Version 3 Works” section on page 9-13
).
VTP version 2 supports the following features that are not supported in version 1:
•
Unrecognized Type-Length-Value (TLV) Support—A VTP server or client propagates configuration
changes to its other trunks even for TLVs it is not able to parse. The unrecognized TLV is saved in
NVRAM.
•
Version-Dependent Transparent Mode—In VTP version 1, a VTP transparent switch inspects VTP
messages for the domain name and version and forwards a message only if the version and domain
name match. Since only one domain is supported in the supervisor engine software, VTP version 2
forwards VTP messages in transparent mode without checking the version.
•
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.