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DES-3010F/G / DES-3018 / DES-3026 Fast Ethernet Switch Manual

The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally

contained in the packet is retained.

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.

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