Port Vlan Id(Pvid); Tagging And Untagging - D-Link DES-3350SR User Manual

Port VLAN ID(PVID)

Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant network device
to another with the VLAN information intact. This allows 802.1Q VLANs to span network devices (and indeed, the entire network
– if all network devices are 802.1Q compliant).
Unfortunately, not all network devices are 802.1Q compliant. These devices are referred to as tag-unaware. 802.1Q devices are
referred to as tag-aware.
Prior to the adoption 802.1Q VLANs, port-based and MAC-based VLANs were in common use. These VLANs relied upon a Port
VLAN ID (PVID) to forward packets. A packet received on a given port would be assigned that port's PVID and then be
forwarded to the port that corresponded to the packet's destination address (found in the switch's forwarding table). If the PVID of
the port that received the packet is different from the PVID of the port that is to transmit the packet, the switch will drop the packet.
Within the switch, different PVIDs mean different VLANs. (remember that two VLANs cannot communicate without an external
router). So, VLAN identification based upon the PVIDs cannot create VLANs that extend outside a given switch (or switch stack).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no VLANs are
defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged packets are assigned the
PVID of the port on which they were received. Forwarding decisions are based upon this PVID, insofar as VLANs are concerned.
Tagged packets are forwarded according to the VID contained within the tag. Tagged packets are also assigned a PVID, but the
PVID is not used to make packet forwarding decisions, the VID is.
Tag-aware switches must keep a table to relate PVIDs within the switch to VIDs on the network. The switch will compare the VID
of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two VIDs are different, the switch will drop
the packet. Because of the existence of the PVID for untagged packets and the VID for tagged packets, tag-aware and tag-unaware
network devices can coexist on the same network.
A switch port can have only one PVID, but can have as many VIDs as the switch has memory in its VLAN table to store them.
Because some devices on a network may be tag-unaware, a decision must be made at each port on a tag-aware device before
packets are transmitted – should the packet to be transmitted have a tag or not? If the transmitting port is connected to a tag-
unaware device, the packet should be untagged. If the transmitting port is connected to a tag-aware device, the packet should be
tagged.

Tagging and Untagging

Every port on an 802.1Q compliant switch can be configured as tagging or untagging.
D-Link DES-3350SR Standalone Layer 3 Switch
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Port Settings
Tagged/None
None
Egress
Forbidden
Advertising can be enabled or disabled
using this pull-down menu. Advertising
allows members to join this VLAN
through GVRP.
Allows an individual port to be specified
as member of a VLAN.
Allows an individual port to be specified
as Tagging. A check in the Tagged field
specifies the port as a Tagging member
of the VLAN. When an untagged packet
is transmitted by the port, the packet
header is changed to include the 32-bit
tag associated with the VID (VLAN
Identifier – see below). When a tagged
packet exits the port, the packet header
is unchanged.
Allows an individual port to be specified
as None. When an untagged packet is
transmitted by the port, the packet
header remains unchanged. When a
tagged packet exits the port, the tag is
stripped and the packet is changed to an
untagged packet.
Egress Member - specifies the port as
being a static member of the VLAN.
Egress Member Ports are ports that will
be transmitting traffic for the VLAN.
These ports can be either tagged or
untagged.
Forbidden Non-Member - specifies the
port as not being a member of the VLAN
and that the port is forbidden from
becoming a member of the VLAN
dynamically.
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