Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 349)

Switching Introduction
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 14.7
The Layer 2 Switching Process
The Layer 2 switching process comprises these related but separate processes:
■ The Ingress Rules
■ The Learning Process
■ The Forwarding Process
■ The Egress Rules
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, 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
All frames, tagged and untagged, that a VLAN-aware switch receives must be classified
into a VLAN. Each received frame is mapped to exactly one VLAN. If an incoming frame is
tagged with a valid VLAN identifier (VID) then that VID is used. If an incoming frame is
untagged or is priority tagged (a tagged frame with a VID of all zeros), then the switch
uses internal VLAN association rules to determine the VLAN it belongs to. The default
settings for the ingress rules are to Admit All Frames, and for Ingress Filtering to be on.
Every port belongs to one or more VLANs so every incoming frame has a VID to show
which VLAN it 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 (by default), frames are admitted only when they have the VID of a VLAN to which
the port belongs. Frames are discarded when they do not have an associated VID
matching the VLAN assigned to a port.
The possible association rules, in order of precedence, are:
■ IP subnet/IPX network classification
■ protocol classification
■ port classification
The default VLAN classification is based upon the port on which the incoming frame
(untagged, or priority tagged) was received. It is possible for an incoming untagged, or
priority tagged, frame to match more than one of the association rules.
Each port on the switch can be configured to be one of two modes:
■ untagged frames - access mode
■ VLAN-tagged frames - trunk mode