D-Link DES-3550 User Manual page 65

DES-3550 Layer 2 Fast Ethernet Switch User's Guide
forwarding table). If the PVID of the port that received the packet is different from the PVID
of the port that is to transmit the packet, the Switch will drop the packet.
Within the Switch, different PVIDs mean different VLANs (remember that two VLANs
cannot communicate without an external router). Therefore, VLAN identification based upon
the PVIDs cannot create VLANs that extend outside a given switch (or switch stack).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use
within the Switch. If no VLANs are defined on the Switch, all ports are then assigned to a
default VLAN with a PVID equal to 1. Untagged packets are assigned the PVID of the port on
which they were received. Forwarding decisions are based upon this PVID, as far as VLANs
are concerned. Tagged packets are forwarded according to the VID contained within the tag.
Tagged packets are also assigned a PVID, but the PVID is not used to make packet
forwarding decisions, the VID is.
Tag-aware switches must keep a table to relate PVIDs within the Switch to VIDs on the
network. The Switch will compare the VID of a packet to be transmitted to the VID of the
port that is to transmit the packet. If the two VIDs are different, the Switch will drop the
packet. Because of the existence of the PVID for untagged packets and the VID for tagged
packets, tag-aware and tag-unaware network devices can coexist on the same network.
A switch port can have only one PVID, but can have as many VIDs as the Switch has memory
in its VLAN table to store them.
Because some devices on a network may be tag-unaware, a decision must be made at each
port on a tag-aware device before packets are transmitted – should the packet to be transmitted
have a tag or not? If the transmitting port is connected to a tag-unaware device, the packet
should be untagged. If the transmitting port is connected to a tag-aware device, the packet
should be tagged.
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 does not 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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