 (Courtesy of Hewlett-Packard Co., Ref. 9.) (RI) field, which specifies the path which that frame is to take through the network. This concept is illustrated in Figure 11.15 (bottom). Up to this point we have been discussing local bridges. A local bridge spans LANs in the same geographical location. A remote bridge spans LANs in different geographic locations. In this case, an intervening WAN (wide area network) is required. The remote bridge consists of two separate devices that are connected by a WAN, affording transport of data frames between the two. This concept is shown in Figure 11.16. As illustrated in Figure 11.16, the LAN data packet/frame is encapsulated by the remote bridge adding the appropriate WAN header and trailer. The WAN transports the data packet/frame to the distant-end remote bridge, which strips the WAN header and trailer, and delivers the data packet/frame to the far-end LAN. Remote bridges typically use proprietary protocols such that, in most cases, remote bridges from different vendors do not interoperate. Bridges are good devices to segment LANs, particularly CSMA/CD LANs. Segmenting breaks up a LAN into user families. It is expected that there is a high community of interest among members of a family, but a low community of interest among different families. There will be large traffic volumes intrasegment and low traffic volumes intersegment. It should be pointed out that routers are more efficient at segmenting than bridges (Ref. 9). A major limitation of bridges is the inability to balance traffic across two or more redundant routes in a network. The existence of multiple paths in a bridged network can prove to be a bad problem. In such a case, we are again faced with the endless route situation as we were with repeaters. One way to avoid the problem is to use the spanning tree algorithm. This algorithm is implemented by having bridges communicate with each other to establish a subset of the actual network topology that is loop-free (often called a tree). The idea, of course, is to