Allied Telesis AR450S Manual de usuario descargar pdf (Pagina 13)

Switching on the AR440S, AR441S and AR450S 3-13

Software Release 2.6.6

C613-03079-00 REV A

Summary of VLAN tagging rules

When designing a VLAN and adding ports to VLANs, the following rules

apply.

1. Each port must belong to at least one static VLAN. By default, a port is an

untagged member of the default VLAN.

2. A port can be untagged for zero or one VLAN. A port that is untagged for

a VLAN transmits frames destined for that VLAN without a VLAN tag in

the Ethernet frame.

3. A port can be tagged for zero or more VLANs. A port that is tagged for a

VLAN transmits frames destined for that VLAN with a VLAN tag,

including the numerical VLAN Identifier of the VLAN.

4. A port cannot be untagged and tagged for the same VLAN.

The Layer 2 Switching Process

The Layer 2 switching process comprises related but separate processes. The

Ingress Rules admit or discard frames based on their VLAN tagging. The

Learning Process learns the MAC addresses and VLAN membership of frames

admitted on each port. The Forwarding Process determines which ports the

frames are forwarded to, and the Quality of Service priority with which they are

transmitted. Finally, the Egress Rules determine for each frame whether VLAN

tags are included in the Ethernet frames that are transmitted. These processes

assume that each station on the extended LAN has a unique data link layer

address, and that all data link layer frames have a header which includes the

source (sender’s) MAC address and destination (recipient’s) MAC address.

The Ingress Rules

When a frame first arrives at a port, the Ingress Rules for the port check the

VLAN tagging in the frame to determine whether it should be discarded or

forwarded to the Learning Process.

The first check depends on whether the Acceptable Frame Types parameter is set

to Admit All Frames or to Admit Only VLAN Tagged Frames. A port that transmits

only VLAN tagged frames, regardless of which VLAN the port belongs to, is

automatically set to Admit Only VLAN Tagged Frames. The user cannot

change this setting. Frames with a null numerical VLAN Identifier (VID) are

VLAN untagged frames, or frames with priority tagging only.

Every frame received by the router must be associated with a VLAN. If a frame

is admitted by the Acceptable Frame Types parameter, the second part of the

Ingress Rules associates each untagged frame admitted with the VID of the

VLAN for which the port is untagged.

Every port belongs to one or more VLANs, and therefore every incoming

frame has a VID to show to which VLAN the frame belongs. The final part of

the Ingress Rules depends on whether Ingress Filtering is enabled for the port. If

Ingress Filtering is disabled, all frames are passed on to the Learning Process,

regardless of which VLAN they belong to. If Ingress Filtering is enabled,

frames are admitted only if they have the VID of a VLAN to which the port

belongs. If they have the VID of a VLAN to which the port does not belong,

they are discarded.

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Allied Telesis AR450S Manual de usuario Pagina 13

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