cording to the MAC source and destination address(es) of each individual data frame. MAC-level broadcasts, however, are propagated through the network by the bridges. A bridge can be used for segmenting and extending LAN coverage. Thus it lowers traffic volume for each segment. A transparent bridge does not modify any part of a message that it forwards. The second bridge is the translation bridge. It is used to connect two dissimilar LANS, such as a token ring to CSMA/CD. In order to do this it must modify the MAC-level header and FCS of each frame it forwards in order to make it compatible with the receiving LAN segment. The MAC addresses and the rest of the data frame are unchanged. Translation bridges are far less common than transparent bridges. The third type of bridge, as shown in Figure 11.15, is the encapsulation bridge. It is also used to connect LANs of dissimilar protocols. But rather than translate the MAC header and FCS fields, it simply appends a second MAC layer protocol around the original frame for transport over the intermediate LAN with a different protocol. There is the destination bridge which strips off this additional layer and extracts the original frame for delivery to the destination network segment. The fourth type of bridge is a source routing bridge. It is commonly used in token ring networks. With source routing bridges, each frame carries within it a route identifier Encapsulation A second MAC header is placed on the frame for transport over a different topology. Source Route Route identifier (RI) field in each frame describes the path. Figure 11.15 The concept of bridging. Top: encapsulation bridge; bottom: source routing bridge. (Courtesy of Hewlett-Packard Co., Ref. 9.) ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS The WAN link is a pipe that is used to carry the LAN packet. There are no changes to the LAN packet. Preamble Ethernet Network Protocol Packet Ethernet Network Protocol Frame Figure 11.16 The concept of remote bridging. The LAN frame/packet is encapsulated in a WAN frame.