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DVANCED
OPICS
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.
When the switch receives a frame, it assigns the frame to the port's
default VLAN if the frame is untagged (determined by the PVID of
the receiving port), or maps it for output to the broadcast domain
associated with the frame's VLAN tag.
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 can connect them
using a Layer 3 switch or a router.
Forwarding Tagged/Untagged Frames
Ports can be assigned to multiple tagged or untagged VLANs.
Each port on the switch is therefore capable of passing tagged or
untagged frames. To forward a frame from a VLAN-aware device
to a VLAN-unaware device, the switch first decides where to
forward the frame, and then strips off the VLAN tag. However, to
forward a frame from a VLAN-unaware device to a VLAN-aware
device, the switch first decides where to forward the frame, and
then inserts a VLAN tag reflecting this port's default VID. The
default PVID is VLAN 1 for all ports, but this can be changed (see
page 2-58 or page 3-39).
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