Cisco 350 Series Administration Manual page 278

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If a frame is VLAN-tagged, a four-byte VLAN tag is added to each Ethernet frame. The tag
contains a VLAN ID between 1 and 4094, and a VLAN Priority Tag (VPT) between 0 and 7.
See Quality of Service for details about VPT.
When a frame enters a VLAN-aware device, it is classified as belonging to a VLAN, based on
the four-byte VLAN tag in the frame.
If there is no VLAN tag in the frame or the frame is priority-tagged only, the frame is
classified to the VLAN based on the PVID (Port VLAN Identifier) configured at the ingress
port where the frame is received.
The frame is discarded at the ingress port if Ingress Filtering is enabled and the ingress port is
not a member of the VLAN to which the packet belongs. A frame is regarded as priority-
tagged only if the VID in its VLAN tag is 0.
Frames belonging to a VLAN remain within the VLAN. This is achieved by sending or
forwarding a frame only to egress ports that are members of the target VLAN. An egress port
may be a tagged or untagged member of a VLAN.
The egress port:
• Adds a VLAN tag to the frame if the egress port is a tagged member of the target
VLAN, and the original frame does not have a VLAN tag.
• Removes the VLAN tag from the frame if the egress port is an untagged member of the
target VLAN, and the original frame has a VLAN tag.
VLAN Roles
VLANs function at Layer 2. All VLAN traffic (Unicast/Broadcast/Multicast) remains within
its VLAN. Devices attached to different VLANs do not have direct connectivity to each other
over the Ethernet MAC layer. Devices from different VLANs can communicate with each
other only through Layer 3 routers. An IP router, for example, is required to route IP traffic
between VLANs if each VLAN represents an IP subnet.
The IP router might be a traditional router, where each of its interfaces connects to only one
VLAN. Traffic to and from a traditional IP router must be VLAN untagged. The IP router can
be a VLAN-aware router, where each of its interfaces can connect to one or more VLANs.
Traffic to and from a VLAN-aware IP router can be VLAN tagged or untagged.
Adjacent VLAN-aware devices exchange VLAN information with each other by using
Generic VLAN Registration Protocol (GVRP). As a result, VLAN information is propagated
through a bridged network.
Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
VLAN Management