Tagging And Untagging; Ingress Filtering; Default Vlans - D-Link xStack DGS-3200-10 User Manual

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Tagged packets are forwarde d according to the VID c ontained within the tag. Ta gged packets are als o assigned a PVID, but the
PVID is not used to make packet-forwarding decisions, the VID is.
Tag-aware switches must keep a tab le to re late PVIDs wit hin the Switch to VIDs on the n etwork. The Sw itch will compare the
VID of a packet to be transmitted to the VID of th e port that is to transmit the packet. If t he two VIDs are d ifferent, the Switch
will drop the packet. Because of the existence of th e PVID for untagged packets and the VID fo r 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 s ome de vices on a network m ay be tag-una ware, a decision must be made at each port on a tag-awa re de vice be fore
packets are tran smitted – sh ould th e p acket to b e tran smitted have a tag or not? If th e tran smitting po rt is co nnected t o a tag -
unaware device, the packet should be untagged. If th e transmitting port is con nected to a tag-a ware 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 th e packet, thus keeping the VLAN i nformation
intact. Other 802.1Q compliant devices on the network to make packet-forwarding decisions can then use the VLAN information
in the tag.
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 no t alter the packet. Thus, all packets receive d 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 ing ress filtering is en abled for a po rt, the Switch will ex amine the VLAN i nformation in th e packet header (i f present) an d
decide whether or not to forward the packet.
If the packet is tag ged with VLAN in formation, the ingress port will first determine if th e ingress port itself is a m ember of the
tagged VLAN. If it is no t, t he p acket will be dropped. If th e i ngress port is a m ember of t he 802.1Q VLAN, t he Switch th en
determines if the destination port is a m ember 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 tagg ing port). Th e switch t hen determines if t he 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 u sed 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.

Default VLANs

The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the Switch to
the "default." As new VLANs are configured in Port-based mode, their respective member ports are removed from the "default."
Packets cannot cross VLANs. If a member of one VLAN wants to connect to another VLAN, the link must be through an external
router.
NOTE: If no VLANs are configured on the Switch, then all packets will be forwarded to any
destination port. Packets with unknown source addresses will be flooded to all ports.
Broadcast and multicast packets will also be flooded to all ports.
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DGS-3200 Series Layer 2 Gigabit Ethernet Managed Switch
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