Perle Systems IOLINK-PRO IOLINK-520 - Manual (Page 14)

Introduction
IOLINK-PRO & 520 Reference Manua — 1.13
The Initial Bridging Process
Each time a IOLINK router is powered up, it will perform extensive hardware and software tests to ensure the integrity of the
unit and its attached LAN and Link interfaces. Upon successful completion of the power-up diagnostics, the IOLINK router
will follow rules to “learn” several aspects of your LAN environment. These rules define what actions are taken under
particular situations.
One of the more important rules employed by the IOLINK router is also a very fundamental part of the bridging process.
This rule dictates how Ethernet Station Addresses are processed by the bridge. The process is outlined below:
Station Address Learning
The IOLINK router performs an important bandwidth-conserving function by a process termed Station Address
Learning. This process determines the location of all active LAN Stations by monitoring the Ethernet frames being
transmitted onto the LAN segments. Once it has learned the location of each station, the remote bridge/router will
not forward those Ethernet frames destined for a station if the receiving station exists on the same LAN. Under
these conditions, the bridge/router will only forward a frame if the location of the destination station has not yet
been learned, or if the location has been determined to exist on the other LAN segment.
To perform this process, the IOLINK router follows the steps outlined below:
Learning Local Addresses
When the bridge/router is powered up, and after completing the power-up diagnostics, it will not immediately begin
forwarding frames between LAN segments. Instead it will listen to local LAN activity in order to learn the location
of each station address on each side of the bridge.
The bridge/router captures each frame and looks at the source address contained within the Ethernet frame. Since
the bridge/router knows which LAN segment the frame was received from, it can determine that this station must be
located on this segment. As a result, it has just learned the location of the station.
This process will continue for the period defined by the Forwarding Delay option, and in this fashion the first stage
of the LAN address table is built.
Forwarding
Once the initial learning process is complete, the bridge/router enters a forwarding mode and examines frames that
may need to be forwarded. The learning process does not stop at this time, however: The bridge/router will continue
learning new stations as they become active on a LAN segment.
Local Destination Addresses
When a frame is received from a station on one segment, the frame is examined for the source address to ensure that
this station has already been entered into the address table. If the source address exists, the Ethernet destination
address is then viewed. The bridge searches the previously built address table for the location of the destination
station. If it is determined that the location of the destination station exists on the same LAN segment (i.e. the
destination address is local and the frame does not need to be forwarded across the bridge to the other LAN
segment), then the bridge will “filter” and discard it.
Initially, the bridge will only recognize those addresses that are local to a specific LAN segment. The bridge will
thereby filter (discard) all local packets and forward all unknown non-local packets to the second segment located on
the outbound port across the bridge.