DES-3010F/DES-3010FL/DES-3010G/DES-3018/DES-3026 Fast Ethernet Switch Manual
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-for-
warding 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 and the destination port transmits it to its attached network segment.
If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own PVID as a VID (if the port is
a tagging port). The switch then determines if the destination port is a member of the same VLAN (has the same VID) as the
ingress port. If it does not, the packet is dropped. If it has the same VID, the packet is forwarded and the destination port transmits
it on its attached network segment.
This process is referred to as ingress filtering and is used to conserve bandwidth within the Switch by dropping packets that are
not on the same VLAN as the ingress port at the point of reception. This eliminates the subsequent processing of packets that will
just be dropped by the destination port.
Default VLANs
The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the Switch to
the "default." As new VLANs are configured in Port-based mode, their respective member ports are removed from the "default."
Packets cannot cross VLANs. If a member of one VLAN wants to connect to another VLAN, the link must be through an external
router.
NOTE: If no VLANs are configured on the Switch, then all packets will be
forwarded to any destination port. Packets with unknown source
addresses will be flooded to all ports. Broadcast and multicast packets will
also be flooded to all ports.
An example is presented below:
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