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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
78 Switch Management and Operating Concepts
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 can then
be used by other 802.1Q compliant devices on the network to
make packet forwarding decisions.
Ports with untagging enabled will strip the 802.1Q tag from all
packets that flow into and out of those ports. If the packet
doesn’t have an 802.1Q VLAN tag, the port will not alter the
packet. Thus, all packets received by and forwarded by an
untagging port will have no 802.1Q VLAN information.
(Remember that the PVID is only used internally within the
switch). Untagging is used to send packets from an 802.1Q-
compliant network device to a non-compliant network device.
Ingress Filtering
A port on a switch where packets are flowing into the switch
and VLAN decisions must be made is referred to as an ingress
port. If ingress filtering is enabled for a port, the switch will
examine the VLAN information in the packet header (if present)
and decide whether or not to forward the packet.
If the packet is tagged with VLAN information, the ingress port
will first determine if the ingress port itself is a member of the
tagged VLAN. If it is not, the packet will be dropped. If the
ingress port is a member of the 802.1Q VLAN, the switch then
determines if the destination port is a member of the 802.1Q
VLAN. If it is not, the packet is dropped. If the destination
port is a member of the 802.1Q VLAN, the packet is forwarded