White Papers

Enabling Null VLAN as the Default VLAN
In a Carrier Ethernet for Metro Service environment, service providers who perform frequent recongurations for customers with changing
requirements occasionally enable multiple interfaces, each connected to a dierent customer, before the interfaces are fully congured.
This presents a vulnerability because both interfaces are initially placed in the native VLAN, VLAN 1, and for that period customers are able
to access each other's networks. Dell Networking OS has a Null VLAN to eliminate this vulnerability. When you enable the Null VLAN, all
ports are placed into it by default, so even if you activate the physical ports of multiple customers, no trac is allowed to traverse the links
until each port is place in another VLAN.
To enable Null VLAN, use the following command.
• Disable the default VLAN, so that all ports belong to the Null VLAN until congured as a member of another VLAN.
CONFIGURATION mode
default-vlan disable
Default: the default VLAN is enabled (no default-vlan disable).
Conguring Native VLANs
Traditionally, ports can be either untagged for membership to one VLAN or tagged for membership to multiple VLANs.
You must connect an untagged port to a VLAN-unaware station (one that does not understand VLAN tags) and connect a tagged port to a
VLAN-aware station (one that generates and understands VLAN tags).
Native VLAN support breaks this barrier so that you can connect a port to both VLAN-aware and VLAN-unaware stations. Such ports are
referred to as hybrid ports. Physical and port-channel interfaces may be hybrid ports.
Native VLAN is useful in deployments where a Layer 2 port can receive both tagged and untagged trac on the same physical port. The
classic example is connecting a voice-over-IP (VoIP) phone and a PC to the same port of the switch. The VoIP phone is congured to
generate tagged packets (with VLAN = VOICE VLAN) and the attached PC generates untagged packets.
NOTE
: You cannot congure an existing switchport or port channel interface for Native VLAN. Interfaces must have no other
Layer 2 or Layer 3 congurations when using the portmode hybrid command or a similar message displays: % Error: Port
is in Layer-2 mode Gi 5/6.
To congure a port so that it can be a member of untagged and tagged VLANs, use the following commands.
1 Remove any Layer 2 or Layer 3 congurations from the interface.
INTERFACE mode
2 Congure the interface for Hybrid mode.
INTERFACE mode
portmode hybrid
3 Congure the interface for Switchport mode.
INTERFACE mode
switchport
4 Add the interface to a tagged or untagged VLAN.
VLAN INTERFACE mode
[tagged | untagged]
988
Virtual LANs (VLANs)