VLAN Configuration
B-2
Virtual LANs (VLANs)
B.2
VLAN CONFIGURATION
VLAN operation on the switch is enabled by default. Therefore, all frames are transferred
internally through the switch with a VLAN tag. This tag may already be on the frame entering the
switch, or added to the frame by the switch. VLAN information already existing on frames
entering the switch is automatically handled by the switch. The switch learns VLAN information
from tagged frames and appropriately switches frames out the proper ports based on this
information. The configuration of VLANs for frames entering the switch without tags must be
made by the user of the switch. This configuration can be made either through the console
interface or via SNMP.
Assigning Ports to VLANs
Before enabling VLANs for the switch, you must first assign each port to the VLAN groups it will
participate in. By default, all ports are assigned to VLAN 1 as untagged ports. You should add a
port as a tagged port (that is, a port attached to a VLAN-aware device) if you want it to carry traffic
for one or more VLANs and the device at the other end of the link also supports VLANs. Then
assign the port at the other end of the link to the same VLANs. However, if you want a port on this
switch to participate in one or more VLANs, but the device at the other end of the link does not
support VLANs, then you must add this port as an untagged port (that is, a port attached to a
VLAN-unaware device).
Port-based VLANs are tied to specific ports. The switch’s forwarding decision is based on the
destination MAC address and its associated port. Therefore, to make valid forwarding and flooding
decisions, the switch learns the relationship of the MAC address to its related port—and thus to the
VLAN—at run-time.
VLAN Classification
When the switch receives a frame, it classifies the frame in one of two ways:
•
If the frame is untagged, the switch assigns the frame to an associated VLAN based on the PVID
of the receiving port.
•
If the frame is tagged, the switch uses the tagged VLAN ID to identify the port broadcast domain
of the frame.
Port Overlapping
Port overlapping can be used to allow access to commonly shared network resources among
different VLAN groups, such as file servers or printers. Note that if you implement VLANs which
do not overlap, but still need to communicate, you must connect them using a router or Layer 3
switch.
