DES-3010F/DES-3010FL/DES-3010G/DES-3018/DES-3026 Fast Ethernet Switch Manual
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.
77
To Next Page
Loading...
