Perle Systems Bridging and the P840 Router, Initial Bridging Process, Station Address Learning

Introduction

Bridging and the P840 Router

The bridge portion of the P840 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 P840 router will resolve the media conflicts that might have otherwise prevented the consolidation of these resources.

The P840 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 P840 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 P840 router. These descriptions will help you understand the concepts of bridging and how the P840 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 P840 router and the advanced functions it may perform for you.

The Initial Bridging Process

Each time a P840 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 P840 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 P840 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 P840 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 P840 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.

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