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17-3
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 17 Configuring VTP
Understanding VTP
When the network is configured with more than the maximum 250 VLANs supported by the Catalyst
2950 switches running the enhanced software image (EI), or 128 VLANs supported by the Catalyst 2950
switches running the standard software image (SI), the switch automatically changes from VTP server
or client mode to VTP transparent mode. The switch then operates with the VLAN configuration that
preceded the one that sent it into transparent mode.
VTP Advertisements
Each switch in the VTP domain sends periodic global configuration advertisements from each trunk port
to a reserved multicast address. Neighboring switches receive these advertisements and update their VTP
and VLAN configurations as necessary.
Note Because trunk ports send and receive VTP advertisements, you must ensure that at least one trunk port
is configured on the switch and that this trunk port is connected to the trunk port of a second switch.
Otherwise, the switch cannot receive any VTP advertisements. For more information on trunk ports, see
the “Configuring VLAN Trunks” section on page 16-13.
VTP advertisements distribute this global domain information:
• VTP domain name
• VTP configuration revision number
• Update identity and update timestamp
• MD5 digest VLAN configuration, including maximum transmission unit (MTU) size for each
VLAN.
• Frame format
VTP advertisements distribute this VLAN information for each configured VLAN:
• VLAN IDs
• VLAN name
• VLAN type
VTP client A VTP client behaves like a VTP server, but you cannot create, change, or delete VLANs on a VTP client.
In VTP client mode, VLAN configurations are not saved in NVRAM.
VTP 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 from
other switches from their trunk interfaces. You can create, modify, and delete VLANs on a switch in VTP
transparent mode. The switch must be in VTP transparent mode when you create extended-range VLANs.
See the
“Configuring Extended-Range VLANs” section on page 16-11.
When the switch is in VTP transparent mode, the VTP and VLAN configurations are saved in NVRAM,
but they are not advertised to other
switches. In this mode, VTP mode and domain name are saved in the
switch running configuration and you can save this information in the switch startup configuration file by
entering the copy running-config startup-config privileged EXEC command.
Table 17-1 VTP Modes (continued)
VTP Mode Description