This section describes how to implement VLANs and describes default VLANs, the unassigned (NULL) VLAN, and brouter ports. This section also summarizes the defaults and rules regarding VLAN creation on the switch.
Devices are factory-configured so that all ports are in a port-based VLAN called the default VLAN. Because all ports are in the default VLAN, the device functions like a Layer 2 device. The VLAN ID of this default VLAN is always 1, and it is always a port-based VLAN. You cannot delete the default VLAN.
Internally, the switch creates a special port-based VLAN called NULL VLAN or unassigned VLAN. This is a place holder VLAN for ports that are not members of any port-based VLAN. When a port is removed from all port-based VLANs, it is added to the NULL VLAN as a port member. Ports can belong to policy-based VLANs as well as to the NULL VLAN. If a frame does not meet the policy criteria and no underlying port-based VLAN exists, the port belongs to the NULL VLAN and the frame is dropped.
Because it is an internal construct, the NULL VLAN cannot be deleted.
A brouter port is actually a one-port VLAN with an IP interface. The difference between a brouter port and a standard IP protocol-based VLAN configured to perform routing is that the routing interface of the brouter port is not subject to the spanning tree state of the port. A brouter port can be in the blocking state for nonroutable traffic and still route IP traffic. Because a brouter port is a single-port VLAN, it uses one VLAN ID. Each brouter port decreases the number of available VLANs by one.