D-Link DGS-3224SR Layer 2 Gigabit Ethernet Switch
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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.
If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own PVID as a
VID (if the port is a tagging port). The switch then determines if the destination port is a member of the same
VLAN (has the same VID) as the ingress port. If it does not, the packet is dropped. If it has the same VID, the
packet is forwarded and the destination port transmits it on its attached network segment.
This process is referred to as
ingress filtering
and is used to conserve bandwidth within the switch by dropping
packets that are not on the same VLAN as the ingress port at the point of reception
.
This eliminates the
subsequent processing of packets that will just be dropped by the destination port.