Data Communications Networking Devices - 4th Ed.pdf

5.1 BRIDGES _____________________________________________________________________ 551Physical versus active topologyIn transparent bridging, a distinction is made between the physical and activetopology resulting from bridged local area networks. This distinction enables theconstruction of a network topology in which inactive but physically constructedroutes can be placed into operation if a primary route should fail and in which theinactive and active routes would form an illegal circular path violating the spanningtree algorithm if both routes were active at the same time.Figure 5.8 a) illustrates one possible physical topology of bridged networks. Thecost C) assigned to each bridge will be discussed later in this section. Figure 5.8 b)illustrates a possible active topology for the physical con®guration shown in Figure5.8 a).Figure 5.8 Physical versus active topology. When transparent bridges are used, theactive topology cannot form a closed loop in the Internet