Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Source Route Bridging Whereas transparent bridging with the spanning tree algorithm assumes that all bridging intelligence is in the bridge, source route bridging assumes that most of the intelligence is in the hosts attached to each network. The name source route bridge is derived from the premise that in the route information field (RIF) each source frame contains the entire route to the destination(Exhibit 3-3-13). If a given host A wants to send a frame to host B the source route communication process works in the following manner: •  Initially the host looks at cached routes for host B. •  If the host route is not cached, a test frame is sent out to determine if host B is on the local ring. •  If the test frame does not locate host B then an explore frame is flooded out all bridge ports to find the best route to host B. •  As each bridge receives the explorer frame, it appends the RIF field with local ring and bridge number information, and forwards the frame to each output port. (This action is intended to cause the original broadcast to traverse every possible connection path.) •  As each explorer frame reaches the end destination (host B) the content of the RIF field is reserved and sent back to the originating station, host A. (Depending on the complexity of the network, tens or hundreds of inverted RIF field frames are returned, each with a different route specified.) •  Upon receipt of each of the broadcast responses host A chooses one of the routes and inserts the corresponding route information for destination B into its source route cache. Properly configured end stations can store multiple routes. Exhibit 3-3-13.  IEEE 802.5 RIF Format Source route bridging was developed by IBM and is the predominant bridging protocol for connecting Token Ring LANs. The operational outline described previously shows that the majority of the source route bridge processor load falls on the end workstation. Performance can vary because route decisions are made by individual workstations. Workstation related performance problems are most common in complex network environments or in networks where processor resources (i.e., memory, disk space, I/O,or CPU capacity) are constricted. The last step of the source route sequence has the originating workstation inserting some number of routes into its route cache. The 802.5 specification does not mandate the criteria for choosing a route, but instead makes several recommendations. Route selection can be based on any one or more of the following metrics: •  The first frame received. •  Minimum hops traversed. •  Largest mean transfer unit path. The metric used to make the route selection is specific to the vendor’s implementation of the source route process. This variability can lead to poor performance if a common metric is not used throughout the network. The heavy load placed on the workstations, combined with hardware limitations, leads to a seven hop limitation, meaning that a real world source route bridge implementation cannot have more than seven bridges cascaded serially. The 802.5 specification allows up to 28 cascaded bridges,but this limit is not achievable. Another distance related issue with source route bridge implementations is that most implementations are based on the LLC2 specification, which specifies a connection oriented link level protocol with a 2 second timeout. The connection oriented sessions in conjunction with the 2 second timeout can lead to time-out problems in networks that use seven hops. This problem is especially prevalent in networks that communicate over WAN links. The LLC1 link level protocol, which is connectionless and therefore has no timeout function, eliminates this problem in transparent bridging. Although source route bridging has a number of limiting factors,it also has several advantages over transparent bridging. Source route bridges make use of explicit routes, which means that the routing information is contained in each frame. This mechanism makes problem resolution easier because complete route information is contained in each frame. The explicit route implementation permits parallel routes to exist, which can be used for load sharing or fault tolerance. Thus far this source route bridge discussion has been limited to bridging between Token Ring LANs. Real world network implementations mix physical and link level protocols. The introduction of distributed applications and integrated business functions requires communication between processors on different LAN types. The advent of new bridges and router/bridges has made possible bridged communications between different LAN media. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. 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