KB.15.15

Special VLAN types

VLAN support and the default VLAN

In the factory default configuration, VLAN support is enabled and all ports on the switch belong

to the port-based, default VLAN (named DEFAULT_VLAN). This places all ports in the switch into

one physical broadcast domain. In the factory-default state, the default VLAN is also the Primary

VLAN.

• You can partition the switch into multiple virtual broadcast domains by configuring one or

more additional VLANs and moving ports from the default VLAN to the new VLANs.

• The switch supports up to 2048 static and dynamic VLANs, with VIDs numbered up to 4094.

You can change the name of the default VLAN, but not its VID, which is always 1.

• You can remove all ports from the default VLAN by placing them in another port-based VLAN,

but this VLAN remains and cannot be deleted from the switch.

The primary VLAN

As certain features and management functions run on only one VLAN in the switch, and because

DHCP and Bootp can run per-VLAN, there is a need for a dedicated VLAN to manage these features

and ensure that multiple instances of DHCP or Bootp on different VLANs do not result in conflicting

configuration values for the switch.

The Primary VLAN is the VLAN the switch uses to run and manage these features and data. In the

factory-default configuration, the switch designates the default VLAN (DEFAULT_VLAN; VID=1) as

the Primary VLAN. However you can designate another static, port-based VLAN as primary.

To summarize, designating a non-default VLAN as primary means that:

• The switch reads DHCP responses on the Primary VLAN instead of on the default VLAN. This

includes such DHCP-resolved parameters as the TimeP server address, Default TTL, and IP

addressing—including the Gateway IP address—when the switch configuration specifies DHCP

as the source for these values.

• The default VLAN continues to operate as a standard VLAN you cannot delete it or change

its VID.

• Any ports not specifically assigned to another VLAN will remain assigned to the Default VLAN,

even if it is the Primary VLAN.

Candidates for Primary VLAN include any static, port-based VLAN currently configured on the

switch.

Protocol-Based VLANs and dynamic (GVRP-learned) VLANs that have not been converted to a

static VLAN cannot be the Primary VLAN. To display the current Primary VLAN, use the CLI show

vlan command.

NOTE: If you configure a non-default VLAN as the Primary VLAN, you cannot delete that VLAN

unless you first select a different VLAN to serve as primary.

If you manually configure a gateway on the switch, it ignores any gateway address received via

DHCP or Bootp.

Introducing tagged VLAN technology into networks running untagged VLANs 57