Allied Telesis Rapier Switch - Manual (Page 46)

46 Rapier Switch Software Reference
Software Release 2.5.1
C613-02025-00 REV B
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 will 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, will
be 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 switch 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 will have a VID to show which VLAN it belongs to. 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.
The default settings for the Ingress Rules are to Admit All Frames, and for
Ingress Filtering to be OFF. This means that if no VLAN configuration has been
done, all incoming frames pass on to the Learning Process, regardless of
whether or not they are VLAN tagged. The parameters for each port’s Ingress
Rules can be configured using the command:
SET SWITCH PORT={port-list|ALL} [ACCEPTABLE={VLAN|ALL}]
[INFILTERING={ON|OFF}] [other-parameters...]
The ACCEPTABLE parameter sets the Acceptable Frame Types parameter, in
the Ingress Rules, which controls reception of VLAN-tagged and VLAN-
untagged frames on the port. If ALL is specified, then the Acceptable Frame
Types parameter is set to Admit All Frames. If VLAN is specified, the
parameter is set to Admit Only VLAN-tagged Frames, and any frame received
that carries a null VLAN Identifier (VID) is discarded by the ingress rules.
Untagged frames and priority-tagged frames carry a null VID. Untagged
frames admitted according to the ACCEPTABLE parameter have the VID of the
VLAN for which the port is untagged associated with them. The