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VLANs are broadcast domains defined within switches to allow control of broadcast, multicast, unicast, and unknown unicast within a Layer 2 device.
VLANs are defined on a switch in an internal database known as the VLAN Trunking Protocol (VTP) database. After a VLAN has been created, ports are assigned to the VLAN.
VLANs are assigned numbers for identification within and between switches. Cisco switches have two ranges of VLANs, the normal range and extended range.
VLANs have a variety of configurable parameters, including name, type, and state.
Several VLANs are reserved, and some can be used for internal purposes within the switch.
VLANs are created on Layer 2 switches to control broadcasts and enforce the use of a Layer 3 device for communications. Each VLAN is created in the local switch's database for use. If a VLAN is not known to a switch, that switch cannot transfer traffic across any of its ports for that VLAN. VLANs are created by number, and there are two ranges of usable VLAN numbers (normal range 1–1000 and extended range 1025–4096). When a VLAN is created, you can also give it certain attributes such as a VLAN name, VLAN type, and its operational state. To create a VLAN, use the following steps.
VTP is a protocol used by Cisco switches to maintain a consistent database between switches for trunking purposes. VTP is not required to create VLANs; however, Cisco has set it up to act as a conduit for VLAN configuration between switches as a default to make administration of VLANs easier. Because of this, you must first either configure VTP with a domain name or disable VTP on the switch. VTP is explained in detail in section "6-4: VLAN Trunking Protocol."
For Catalyst 4000 and 6000 switches running IOS Supervisor 12.1(8a) or above (native IOS), you can configure the VTP parameters in global configuration mode as well.
Specify a VTP name:
set vtp domain domain-name
(vlan) vtp domain domain-name
(global) vtp domain domain-name
By default, the VTP is in server mode and must be configured with a domain name before any VLANs can be created. These commands specify the VTP domain name. For IOS switches, you enter vlan database mode, (vlan), by entering the command vlan database, at the privileged-level prompt.
The global configuration command vtp domain is not available on all switches that run IOS.
Disable VTP synchronization:
set vtp mode transparent