Tagging And Untagging; Ingress Filtering - D-Link DGS-3312SR Release 3.5 Manual

DGS-3312SR Stackable Gigabit Layer 3 Switch
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.
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.
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