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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
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 5-10. Adding an IEEE 802.1Q Tag
Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q
compliant network device to another with the VLAN information intact. This allows 802.1Q VLANs to
span network devices (and indeed, the entire network – if all network devices are 802.1Q compliant).
Unfortunately, not all network devices are 802.1Q compliant. These devices are referred to as tag-
unaware. 802.1Q devices are referred to as tag-aware.
Prior to the adoption 802.1Q VLANs, port-based and MAC-based VLANs were in common use. These
VLANs relied upon a Port VLAN ID (PVID) to forward packets. A packet received on a given port would
be assigned that port’s PVID and then be forwarded to 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
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