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14-6
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 14 Configuring VLANs
Configuring Normal-Range VLANs
• The switch supports 64 spanning-tree instances. If a switch has more active VLANs than supported
spanning-tree instances, spanning tree can be enabled on 64 VLANs and is disabled on the
remaining VLANs. If you have already used all available spanning-tree instances on a switch,
adding another VLAN anywhere in the VTP domain creates a VLAN on that switch that is not
running spanning-tree. If you have the default allowed list on the trunk ports of that switch (which
is to allow all VLANs), the new VLAN is carried on all trunk ports. Depending on the topology of
the network, this could create a loop in the new VLAN that would not be broken, particularly 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 spanning 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 Chapter 12,
“Configuring RSTP and 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 14-6
You access config-vlan mode by entering the vlan vlan-id global configuration command.
• VLAN Configuration in VLAN Configuration Mode, page 14-6
You access VLAN 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 (Table 14-2) or enter multiple commands to configure the VLAN. For more
information about commands available in this mode, refer to the vlan global configuration command
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 14-12.
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 (Table 14-2) 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 prompt appears.