Sunday, May 16, 2010

Creation of an Ethernet VLAN

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.

Configure VTP.

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."

NOTE

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:

COS
set vtp domain domain-name

IOS
(vlan) vtp domain domain-name

-OR-

(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.

NOTE

The global configuration command vtp domain is not available on all switches that run IOS.

-OR-

Disable VTP synchronization:

COS
set vtp mode transparent

IOS
(vlan) vtp transparent

-OR-

(global) vtp mode transparent



Another option is to disable VTP synchronization of the databases. Disabling it enables you to manage your local VTP database without configuring and relying on VTP. 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.

NOTE

The global configuration command vtp mode transparent is not available on all switches that run IOS.

-OR-

Disable VTP:

COS
set vtp mode off

IOS
N/A



With the introduction of COS version 7.1.1, an option now exists to disable VTP completely. Use the command set vtp mode off to turn off VTP. After doing so, you can administer the local VTP database.

Create the VLAN.

VLANs are created by number. The two ranges of VLANs are as follows:

The standard range consists of VLANs 1 to 1000.

The extended range consists of VLANs 1025 to 4096.

Extended VLANs are currently supported only on switches running COS software version 6.1 or greater. When you create a VLAN, you have many options to consider. Many options are valid only for FDDI and Token Ring VLANs. Some of the items configured deal with options, such as private VLANs, which are discussed in other sections in this book. VLANs are created using the set vlan command for COS devices or with the vlan command in vlan database mode for IOS switches. For Ethernet VLANs, you can also configure the standard

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