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DES-3010F/DES-3010FL/DES-3010G/DES-3016/DES-3018/DES-3026 Fast Ethernet Switch Manual
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Figure 7- 2. IEEE 802.1Q Tag
The EtherType and VLAN ID are inserted after the MAC source address, but before the original
EtherType/Length or Logical Link Control. Because the packet is now a bit longer than it was originally, the
Cyclic Redundancy Check (CRC) must be recalculated.
Figure 7- 3. Adding an IEEE 802.1Q Tag
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