D-Link DES-3326SRM Product Manual - Page 146

Tagging and Untagging, Ingress Filtering

Page 146 highlights

D-Link DES-3326S Layer 3 Switch Switch will drop the packet. Because of the existence of the PVID for untagged packets and the VID for tagged packets, tagaware 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 tagunaware 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. 136

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D-Link DES-3326S Layer 3 Switch
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.
136