Data Communications Networking Devices - 4th Ed.pdf

5.1 BRIDGES _____________________________________________________________________ 549MAC frames. The bridge then encapsulates one or more LAN frames into thebridging protocol frame used for communications over the wide area network.Since the LAN frame is wrapped in another protocol, we say the LAN frame istunneled within the WAN protocol. At the opposite end of the wide area networkthe distant remote bridge performs a reverse operation, removing the WANprotocol header and trailer from each frame.FabricationSome bridges are manufactured as standalone products. Such devices can beconsidered as `plug and play', as you simply connect the bridge to the media andpower it on. Other bridges are manufactured as adapter cards for insertion into thesystem unit of a personal computer, workstation, or reduced instruction set computerRISC). Through the use of software developed in conjunction with hardwareyou may obtain more ¯exibility in the use of this type of bridge than astandalone device whose software is ®xed in ROM.Routing methodsThe routing capability of a bridge governs its abilty to interconnect local areanetworks. Two types of routing methods used by bridges are transparent andsource routing. Transparent bridging was originally developed to support theinterconnection of Ethernet networks and is based upon the use of the spanningtree algorithm to prevent closed loops from allowing frames to continuouslycirculate a group of connected networks. Source routing was developed to supportbridging of Token-Ring networks and uses the routing information ®eld RIF) in aToken-Ring frame to create and use a route between source and destination. Thisenables source routing bridging to support a closed loop network structure. Athirdtype of bridge is a source route-transparent bridge which enables Ethernet andToken-Ring networks to be connected while supporting the appropriate routingtechnique on each side of the bridge commensurate with the type of network thebridge is connected to.To illustrate the problem associated with a closed loop architecture and the needfor a spanning tree algorithm, consider Figure 5.7 which illustrates the connectionof three Ethernet local area networks by the use of three two-port bridges.In this example the interconnected networks form a circular or loop topology.Since a transparent bridge views workstations as being connected to either port 1 orport 2, a circular or loop topology will create problems. Those problems can resultin an unnecessary duplication of frames which will not only degrade the overalllevel of performance of the interconnected network but quite possibly confuse endstations. For example, consider a frame whose source address is Aand whosedestination address is F. Both bridge 1 and bridge 2 will forward the frame.Although bridge 1 will forward the frame to its appropriate network using the mostdirect route, the frame will also be forwarded by bridge 3 to Ethernet 2, resulting ina duplicate frame arriving at workstation F. At workstation F a mechanism wouldbe required to reject duplicate frames. Even if such a mechanism is available, the