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DGS-3024
Инструкция по эксплуатации Страница: 48

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DGS-3024
Инструкция по эксплуатации Страница: 48

D-Link DGS-3024 — настройки VLAN и управление портами в сетевых коммутаторах [48/184]

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D-Link DGS-3024 [48/184] Tagging and untagging

D-Link DES-3350SR Standalone Layer 3 Switch

Advertising can be enabled or disabled

using this pull-down menu. Advertising

allows members to join this VLAN

through GVRP.

Port Settings

Allows an individual port to be specified

as member of a VLAN.

Tagged/None

Allows an individual port to be specified

as Tagging. A check in the Tagged field

specifies the port as a Tagging member

of the VLAN. When an untagged packet

is transmitted by the port, the packet

header is changed to include the 32-bit

tag associated with the VID (VLAN

Identifier – see below). When a tagged

packet exits the port, the packet header

is unchanged.

None

Allows an individual port to be specified

as None. When an untagged packet is

transmitted by the port, the packet

header remains unchanged. When a

tagged packet exits the port, the tag is

stripped and the packet is changed to an

untagged packet.

Egress

Egress Member - specifies the port as

being a static member of the VLAN.

Egress Member Ports are ports that will

be transmitting traffic for the VLAN.

These ports can be either tagged or

untagged.

Forbidden

Forbidden Non-Member - specifies the

port as not being a member of the VLAN

and that the port is forbidden from

becoming a member of the VLAN

dynamically.

Port VLAN ID(PVID)

Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant network device

to another with the VLAN information intact. This allows 802.1Q VLANs to span network devices (and indeed, the entire

network – if all network devices are 802.1Q compliant).

Unfortunately, not all network devices are 802.1Q compliant. These devices are referred to as tag-unaware. 802.1Q devices are

referred to as tag-aware.

Prior to the adoption 802.1Q VLANs, port-based and MAC-based VLANs were in common use. These VLANs relied upon a Port

VLAN ID (PVID) to forward packets. A packet received on a given port would be assigned that port’s PVID and then be

forwarded to the port that corresponded to the packet’s 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, insofar 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.

D-Link DGS-3024 — настройки VLAN и управление портами в сетевых коммутаторах [48/184]

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