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An Ethernet packet that contains a VLAN ID is called a tagged packet. An Ethernet
packet without a VLAN ID is called an untagged packet. Typically, all packets leave
untagged, unless tagged by the adapter prior to arriving at the switch port.
Egress and Ingress Rules. Egress rules determine which frames can be transmitted
out of a port, based on the Egress List of the associated VLAN. Each VLAN has an
Egress List that specifies the ports out of which frames can be forwarded, and speci-
fies whether the frames will be transmitted as tagged or untagged frames.
Ingress rules are a means of filtering out undesired traffic on a port. When Ingress Fil-
tering is enabled, a port determines if a frame can be processed based on whether the
port is on the Egress List of the VLAN associated with the frame.
When an untagged packet arrives at the switch port, the switch will write a VLAN ID
into the header of the frame according to the PVID (port VLAN) definition. Typically,
most switches today have all ports set to a default PVID of 1. When a tagged frame
arrives at a switch port, the tag is respected.
A VID defines the member of a port group. A packet can only travel inside a member
port when the member port is part of a VID port group. Different VID groups are not
visible to one another.
Tag-Based VLAN Overview. The figure below shows the breakdown of the VLAN tag
field.
Figure 17.
VLAN tag field
