Data Communications Networking Devices - 4th Ed.pdf

542 ________________________________________________ LAN INTERNETWORKING DEVICESonto the networks attached to ports 1, 2, and 3. Thus, the frame adversely affectsthe performance on the networks connected to ports 1 and 3 as it precludes othertransmissions on those networks for the duration of the frame that will be discarded,since its intended recipient is not on either network. In addition to ¯oodingthe frame, the bridge also updates its address/port table as it noted that the stationwith source address Ais on port 0. This update is shown in the second entry at thebottom of Figure 5.2.Next, let's assume the workstation with address G transmits a frame to thestation whose address is A. The bridge searches its address/port table, noting thataddress Ais associated with port 0. Thus, the bridge forwards the frame onto thenetwork connected to port 0. Note that the forwarding process depends upon theability of a bridge to ®rst learn the addresses of network devices. Thus, theforwarding process is sometimes referred to as a backward or reverse learningprocess. Returning to the bridge operation, once it notes address Ais on port 0 andforwards the frame, it also notes that address G is not in the address/port table andupdates the table. This update is indicated in the third entry at the bottom ofFigure 5.2.In addition to maintaining MAC addresses and their associated ports, a bridgetime-stamps each entry. The time stamp is used to purge aged entries and enablesthe ®nite memory of the bridge to hold the most recently noted addresses. Sincemany workstations have signi®cant periods of network inactivity, it is quite commonfor entries to be purged from a bridge's address/port table. However, oncepurged, the ®rst frame with a destination address no longer in the address/porttable will result in the bridge ¯ooding the frame. Thus, ¯ooding can be consideredas a process associated with the use of bridges that will continue to occur periodicallylong after a bridge is powered on. This means that in addition to providing amechanism to transmit frames to locations whose destinations are not presentlyknown, ¯ooding places an additional level of utilization on networks.Filtering and forwardingThe process of examining each frame is known as ®ltering. The ®ltering rate of abridge is directly related to its level of performance. That is, the higher the ®lteringrate of a bridge the lower the probability it will become a bottleneck to networkperformance. Asecond performance measurement associated with bridges istheir forwarding rate. The forwarding rate is expressed in frames per second anddenotes the maximum capability of a bridge to transmit traf®c from one network toanother.Types of bridgesThere are two general types of bridgesÐtransparent and translating. Each type ofbridge can be obtained as a local or remote device, with a remote device including awide area network interface as well as the ability to convert frames into a WANtransmission protocol.