Tagging And Untagging - Enterasys VH-2402-L3 Management Manual

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, in so far 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.
Ports with tagging enabled will put the VID number, priority
and other VLAN information into the header of all packets
that flow into and out of it. If a packet has previously been
tagged, the port will not alter the packet, thus keeping the
VLAN information intact. The VLAN information in the tag
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