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DES-3226 NWay Standalone Fast Ethernet Switch User’s Guide
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, 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 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.
If the packet is tagged with VLAN information, the ingress port will first determine if the ingress port
itself is a member of the tagged VLAN. If it is not, the packet will be dropped. If the ingress port is a
member of the 802.1Q VLAN, the switch then determines if the destination port is a member of the
802.1Q VLAN. If it is not, the packet is dropped. If the destination port is a member of the 802.1Q
VLAN, the packet is forwarded and the destination port transmits it to its attached network segment.
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