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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter17 Configuring VLANs
Configuring Normal-Range VLANs
is to allow all VLANs), the new VLAN is carried on all trunk ports. Depending o n the topo l ogy of
the network, this could create a loop in the new VLAN that would not be broken, particularl y if there
are several adjacent switches that all have run out of spanning-tree instances. You can prevent this
possibility by setting allowed lists on the trunk ports of switches that have used up their allocation
of spanning-tree instances.
If the number of VLANs on the switch exceeds the number of supported spanni ng tree instances, we
recommend that you configure the IEEE 802.1S Multiple STP (MSTP) on your switch to map
multiple VLANs to a single STP instance. For more information about MSTP, see Chapter15,
“Configuring MSTP.”
VLAN Configuration Mode Options
You can configure normal-range VLANs (with VLAN IDs 1 to 1005) by using these two configuration
modes:
•VLAN Configuration in config-vlan Mode, page 17-6
You access config-vlan mode by entering the vlan vlan-id global configuration command.
•VLAN Configuration in VLAN Configuration Mode, page 17-6
You access VLAN database configuration mode by entering the vlan database privileged EXEC
command.

VLAN Configuration in config-vlan Mode

To access config-vlan mode, enter the vlan global configuration command with a VLAN ID. Enter a new
VLAN ID to create a VLAN or with an existing VLAN ID to modify the VLAN. You can use the default
VLAN configuration (Table17-2) or enter multiple commands to configure the VLAN. For more
information about commands available in this mode, refer to the vlan global configuration com mand
description in the command reference for this release. When you have finished the configuration, you
must exit config-vlan mode for the configuration to take effect. To display the VLAN configuration,
enter the show vlan privileged EXEC command.
You must use this config-vlan mode when creating extended-range VLANs (VLAN IDs greater than
1005). See the “Configuring Extended-Range VLANs” section on page 17-12.

VLAN Configuration in VLAN Configuration Mode

To access VLAN configuration mode, enter the vlan database privileged EXEC command. Then enter
the vlan command with a new VLAN ID to create a VLAN or with an existing VLAN ID to modify the
VLAN. You can use the default VLAN configuration (Tabl e 17-2 ) or enter multiple commands to
configure the VLAN. For more information about keywords available in this mode, refer to the vlan
VLAN configuration command description in the command reference for this release. When you have
finished the configuration, you must enter apply or exit for the configuration to take effect. When you
enter the exit command, it applies all commands and updates the VLAN database. VTP messages are
sent to other switches in the VTP domain, and the privileged EXEC mode p rom pt a ppea rs .