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Software Configuration Guide—Release 12.2(25)SG
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Chapter10 Understanding and Conf iguring VLANs, VTP, and VMPS
VLAN Trunking Protocol
Understanding the VTP Domain
A VTP domain is made up of one or more interconnected net work devices that share the same VTP
domain name. A network device can be configured to be in only one VTP domain. You make global
VLAN configuration changes for the domain using either the command-line interface (CLI) or Simple
Network Management Protocol (SNMP).
By default, the Catalyst 4500 series switch is in VTP server mode and is in the no-management domain
state until the switch receives an advertisement for a domain over a trunk link or you configure a
management domain. You cannot create or modify VLANs on a VTP server until the management
domain name is specified or learned.
If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name
and the VTP configuration revision number. The switch ignores advertisements with a different
management domain name or an earlier configuration revision numb er.
If you configure the switch as VTP transparent, you can create and modify VLANs, but the changes
affect only the individual switch.
When you make a change to the VLAN configuration on a VTP server, the change is propagated to all
network devices in the VTP domain. VTP advertisements are transmitted out all Inter-Switch Link (ISL)
and IEEE 802.1Q trunk connections.
VTP maps VLANs dynamically across multiple LAN types with unique names and internal index
associations. Mapping eliminates unnecessary device administration f or network administrators.
Understanding VTP Modes
You can configure a Catalyst 4500 series switch to operate in any one of these VTP modes:
•Server—In VTP server mode, you can create, modify, and delete VLANs and specify other
configuration parameters (such as VTP version and VTP pruning) for the entire VTP do main. VTP
servers advertise their VLAN configuration to other network devices in the same VTP domain and
synchronize their VLAN configuration with other network devices based on adverti sements received
over trunk links. VTP server is the default mode.
•Client—VTP clients behave the same way as VTP servers, but you cannot create, change, or delete
VLANs on a VTP client.
•Transparent—VTP transparent network devices do not participate in VTP. A VTP transparent
network device does not advertise its VLAN configuration and does not synchroniz e its VLAN
configuration based on received advertisements. However, in VTP version 2, transparent network
devices do forward VTP advertisements that they receive on their trunking LAN interfaces.
Note Catalyst 4500 series switches automatically change from VTP server mode to VTP client mode if the
switch detects a failure while writing configuration to NVRAM. If this happens, the switch cannot be
returned to VTP server mode until the NVRAM is functioning.
Understanding VTP Advertisements
Each network device in the VTP domain sends periodic advertisements out each trunk ing LAN interface
to a reserved multicast address. VTP advertisements are received by neighboring network devices, which
update their VTP and VLAN configurations as necessary.