Data Communications Networking Devices - 4th Ed.pdf

546 ________________________________________________ LAN INTERNETWORKING DEVICESFor readers familiar with Ethernet bridging, you are probably aware of a constraintreferred to as the spanning tree path which precludes the ability to form a closedloop when bridging. The reason for the spanning tree is to prevent the continuousforwarding of frames in a circular manner that would adversely affect networkperformance. While the spanning tree algorithm would require one of the twobridges shown in Figure 5.4 to be placed into a standby state of operation, since thetwo bridges are considered to be selectively forwarding by address, this is notrequired. That is, by forwarding only over single paths the integrity of the spanningtree is maintained on the Ethernet network. Later in this section we will discuss theoperation of the spanning tree algorithm in detail.Multiple port supportThe multiple port support capability of a bridge is related to its local and wide areanetwork media interface support. Some bridges support additional ports beyondthe two that make up a basic bridge. Doing so enables a bridge to provideconnectivity between three or more local area networks.Figure 5.5 illustrates one potential use of a multiple port bridge to link anEthernet network to two Token-Ring networks.Figure 5.5 Using a multiport bridge. Through the use of a multiport bridge you canconnect three or more local area networks