Ingress Rules; Learning Process - Allied Telesis SwitchBlade x3100 Series Manual

Introduction
4.2 Switching
4.2.1 Overview
As a layer 2 switching device, the Allied Telesis SBx3112 ensures data packets arrive at their proper destination
by using:
VLAN - This is a software-defined subnetwork that allows devices to be grouped into one logical broadcast
•
domain.
MAC address - The MAC address uniquely identifies each hardware device attached to the network.
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The layer 2 switching process includes four separate but related processes:
Ingress Rules admit or discard frames based on their VLAN tagging.
1.
Learning Process learns the MAC addresses for each VLAN as frames are admitted to each interface.
2.
Forwarding Process determines which interfaces the frames are forwarded to.
3.
Egress Rules determine for each frame whether VLAN tags are included in the Ethernet frames that are
4.
Transmitted.
Since this is layer 2, the learning process assumes that each host on the extended LAN has a unique data link
layer address, and all data link layer frames have a header which includes the source (sender) MAC address and
destination (receiver) MAC address.

4.2.2 Ingress Rules

When a frame first arrives at a port, the Ingress Rules for the port will check the VLAN tagging in the frame to
determine whether it will be discarded or forwarded to the Learning Process.
Every frame received by the switch must be associated with a VLAN. If a received frame is untagged, then the
port's untagged VLAN Identifier (VID) will be associated with the received frame. Since every port belongs to
one or more VLANs, every incoming frame will have a VID to indicate which VLAN it belongs to.
The Ingress Rule will check whether the port, in which the frame was received on, belongs to the VLAN indi-
cated by the received frame's VID. If the port is not a member of the VLAN, then the frame will be discarded;
otherwise, the frame will be passed on to the Learning Process.

4.2.3 Learning Process

When a layer 2 ethernet switch first receives frames, the switch floods the data packets. The Learning Process
uses an adaptive learning algorithm -- sometimes called backward learning -- to discover the location (port) of
each host on the extended LAN and ensure frames are sent to their destination as efficiently as possible.
All frames admitted by the Ingress Rules on any port are passed on to the Learning Process, where the frame's
source MAC address and numerical (VID) are compared with entries in the Forwarding Database for the VLAN
(also known as a MAC address table, or a forwarding table) maintained by the switch. The Forwarding Database
contains one entry for every unique host MAC address the switch knows in each VLAN.
4-2
Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
Overview