Specifications

Introduction
13
Bridging and the Ethernet Remote Bridge/Router
The bridge portion of the Ethernet remote bridge/router is an Ethernet Media Access Control (MAC) level bridge providing
an efficient means of interconnecting IEEE 802.3 Local Area Networks supporting a choice of standard Ethernet (10Base5),
Thin Ethernet (10Base2) and Twisted Pair (10BaseT) interfaces. With the support of these industry-standard LAN interface
technologies, the Ethernet remote bridge/router will resolve the media conflicts that might have otherwise prevented the
consolidation of these resources.
The Ethernet remote bridge/router will also fit right into those environments that may require more than one bridge by using
the IEEE 802.1D Spanning Tree Protocol. With this protocol, the Ethernet remote bridge/router will perform automatic
network reconfiguration in the event of a link failure to one of the LAN segments. This provides maximum availability of the
attached LAN services.
Immediately following are several short descriptions of LAN bridging operations specific to the Ethernet remote
bridge/router. These descriptions will help you understand the concepts of bridging and how the Ethernet remote
bridge/router performs these functions.
The remaining sections of this document describe how these functions are performed and configured. You are urged to
spend the small amount of time necessary to familiarize yourself with the Ethernet remote bridge/router and the advanced
functions it may perform for you.
The Initial Bridging Process
Each time a Ethernet bridge/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
Ethernet bridge/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 Ethernet bridge/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 Ethernet bridge/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 Ethernet bridge/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.