D-Link DES-3326 User Manual - page 72
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
58
Switch Management and Operating Concepts
the port that corresponded to the packet’s destination address
(found in the switch’s 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). So, 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, in so 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,