Allied Telesis AR450S Uživatelský manuál stáhnout pdf (Strana 8)

3-8 AR400 Series Router Software Reference

Software Release 2.6.6

C613-03079-00 REV A

The VLAN Identifier values that have specific meaning are listed in Table 3-3

on page 3-8.

Ethernet packets which contain a VLAN tag are referred to as tagged frames,

and switch ports that transmit tagged frames are referred to as tagged ports.

Ethernet packets which do not contain the VLAN tag are referred to as

untagged frames, and switch ports that transmit untagged frames are referred

to as untagged ports. VLANs can consist of simple logical groupings of

untagged ports, in which the ports receive and transmit untagged packets.

Alternatively, VLANs can contain only tagged ports, or a mixture of tagged

and untagged ports.

Switch ports on the router are VLAN aware. They can accept VLAN tagged

frames, and support the VLAN switching required by such tags. A network

can contain a mixture of VLAN aware devices, for example, other 802.1q-

compatible routers, and VLAN unaware devices, for example, workstations

and legacy devices that do not support VLAN tagging. The router can be

configured to send VLAN tagged or untagged frames on each switch port,

depending on whether the devices connected to the port are VLAN aware. By

assigning a port to two different VLANs, to one as an untagged port and to

another as a tagged port, it is possible for the port to transmit both VLAN

tagged and untagged frames.

A VID is associated with every frame admitted on a switch port. If a frame

arrives on a tagged port, the associated VID is determined from the VLAN tag

the frame had when it arrived. If a frame arrives on an untagged port, it is

associated with the VID of the VLAN for which the incoming port is untagged.

When the router forwards a frame over a tagged port, it adds a VLAN tag to

the frame. When the router forwards the frame over an untagged port, it

transmits the frame as a VLAN untagged frame, not including the VID in the

frame.

CFI 1 bit The Canonical Format Indicator (CFI flag) is used to indicate

whether all MAC address information that may be present

in the MAC data carried by the frame is in canonical format.

VID 12 bits The VLAN Identifier (VID) field uniquely identifies the VLAN

to which the frame belongs.

Table 3-3: Reserved VID values.

VID value

(hexadecimal)

Meaning and use of reserved VID values

0 The null VLAN ID. Indicates that the tag header contains only user

priority information; no VLAN Identifier is present in the frame. This

VID value must not be configured in any Forwarding Database entry,

or used in any management operation. Frames that contain the null

VLAN ID are also known as priority-tagged frames.

1 The default VID value used for classifying frames on ingress through

an untagged switch port.

FFF Reserved for implementation use. This VID value must not be

configured in any Forwarding Database entry, used in any

management operation, or transmitted in a tag header.

Table 3-2: Fields in the Ethernet frame for QoS and VLAN switching.

Field Length Meaning and use

1 2 3 4 5 6 7 8 9 10 11 12 13 ... 47 48

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