51-20-36 Bridging Strategies for LAN Internets Previous screen Nathan J. Muller Payoff As corporations continue to move away from centralized computing to distributed, peer-to- peer arrangements, the need to share files and access resources across heterogeneous networks becomes all the more necessary. The need to interconnect dissimilar host systems and LANs may arise from normal business operations or as the result of a corporate merger or acquisition. Whatever the justification, internetworking is becoming ever more important, and the interconnection device industry will grow for the rest of the decade. Introduction The devices that facilitate the interconnection of host systems and LANs fall into the categories of repeaters , bridges, routers, and gateways. Repeaters are the simplest devices and are used to extend the range of LANs and other network facilities by boosting signal strength and reshaping distorted signals. Gateways are the most complex devices; they provide interoperability between applications by performing processing-intensive protocol conversions. In the middle of this “complexity spectrum” are bridges and routers. At the risk of oversimplification, traditional bridges implement basic data-level links between LANs that use identical protocols; traditional routers can be programmed for multiple network protocols, thereby supporting diverse types of LANs and host systems over the same WAN facility. However, in many situations the use of routers is overkill and needlessly expensive; routers cost as much as $75,000 for a full-featured, multiport unit, compared with $6,000 to $30,000 for most bridges. The price difference is attributable to the number of protocols supported, the speed of the Central Processing Unit, port configurations, WAN interfaces, and network management features. Some vendors bundle selected functions of both devices into the same unit, permitting concurrent bridging and routing at a reasonable cost. Bridge and router applications are summarized in Exhibit 1. A Comparison of Bridge and Router Applications and Costs Bridge Applications Router Applications -------------------- --------------------- Previous screen Best for point-to-point Accommodate several data links and and simple mesh topologies. can exploit complex mesh topologies in cases of link failure and congestion. Easier to install and main- tain than routers. Support multiple network and router layer protocols at the same time. Operate independently of higher-level protocols. Offer advanced administration and control services based on network and Offer a flexible method for subnetwork addresses. filtering traffic according to source-destination add- resses, protocol type, and application. ----------------------------------------------------------------------------- Cost: $6,000 to $30,000 Cost: As much as $75,000 _____________________________________________________________________________ Although many companies are implementing LAN/WAN networks based on such vendor-neutral protocols as the transmission control protocol and Internet protocol (TCP/IP) and Open Systems Interconnection, these same companies have considerable investments in System Network Architecture equipment and applications. With many vendors providing connectivity to IBM Corp.'s Token Ring LANs and the SNA environment, it is worthwhile to review IBM's method of bridging and contrast it with the available alternatives. Connectivity Concerns IBM System Network Architecture is still a dominant architecture, but it is no longer able to meet the networking demands of the majority of users. Consequently, its host- controlled, hierarchical structure, with all information flowing through a central point, is rapidly being displaced by distributed computing and peer-to-peer networking over LANs. Although IBM very effectively addresses these needs with its Token Ring LANs, its preferred method for interconnecting them—source routing—for a variety of reasons is unsuitable for building large networks. Nor does source routing support other popular internetworking protocols (e.g., TCP/IP and Novell, Inc.'s IPX/SPX), making it unsuitable for multiprotocol networks. While users appreciate LANs for their efficiencies and economies, older SNA equipment still provides dependable service and may not yet be fully depreciated. Therefore, another problem faced by users is how best to eliminate parallel networks by integrating Synchronous Data Link Control and binary synchronous communications (BSC) serial SNA protocols with LAN traffic on a single network. Bridging Methods Several bridging methods are currently available: source route bridging, preferred by IBM; transparent bridging, a basic method of LAN interconnection supported by most bridge makers; and Source Routing Transparent, a relatively new standard that allows source routing and transparent bridging to be used together on the same network. Source Routing Previous screen Source route bridging is a method of internetworking Token Ring LANs that uses a process called route discovery to find the optimal path for communications between end stations. The route between end stations is discovered using “explorer packets” that are sent between the source and destination end stations. When the explorer packet reaches its destination, the end station responds by issuing a packet containing the Routing Information. If multiple routes are available, this packet is sent back to the source over all of the routes. The originating station selects as the best route the one with the fewest hops to the destination station. One problem with Source routing is that in mesh networks it creates a significant amount of overhead, which can bog down network performance. The amount of overhead increases as more stations and links are added to the network. Because the end-stations are involved in route selection , they may not have up-to- date knowledge of the best path, especially if the network is temporarily congested. In not being able to implement adaptive routing, Source routing bridges are not able to dynamically reroute traffic around failed links. To do this, a new route discovery sequence must be initiated. Source routing bridges also cannot balance the traffic load in response to congestion. Transparent Bridging Transparent bridging originated in the Ethernet environment. It enables stations, regardless of location, to communicate as if they were on the same LAN. In a process called filtering, the bridge looks at the destination address to see if it is listed in the table of source addresses. If not, the packet is sent over the bridge to the next LAN. If a match is found, the bridge simply ignores the packet. In a process called learning, the bridge examines all the packets originating on the LAN to build and update its table of source addresses. A table is maintained for each LAN connected to the bridge. The tables are updated when new packets are detected or when addresses expire from nonuse after a specified time. If a packet contains an address that has not yet been learned, it is sent out over all active links. The best path is determined by an industry-standard spanning tree algorithm, based on such factors as the number of hops from the designated root bridge and speed of the links. Any redundant paths are put in standby mode and used only in case of primary link failure. Source Routing Transparent The source routing transparent (SRT) bridging method combines Source routing and transparent bridging, permitting the data of both to be passed over the same network. With source routing transparent (SRT), the routing information field indicator is used to distinguish between frames using Source routing and frames using transparent bridging. Transparent bridges, including those supporting Ethernet, do not alter the routing indicator, but Source routing bridges do change the routing indicator, setting it to 1. By inspecting the routing indicator, the source routing transparent (SRT)-compliant device can determine whether the frame requires transparent bridging or Source routing (Exhibit 2). In supporting both bridging methods, Source Routing Transparent-compliant devices eliminate the need for multiple types of internetworking equipment and separate network facilities. Previous screen The SRT Bridge Is Capable of Both Source Routing and Transparent Bridging Ramifications of SRT In several key areas, Source Routing Transparent enhances the capabilities of source routing and transparent bridging, providing compelling advantages to users. Route Discovery Transparent bridging employs a simple method of forwarding packets and can learn other locations based on newly encountered addresses. With Source routing, the originating device sends out explorer packets to discover the route between source and destination end stations. source routing transparent (SRT) not only supports both methods, it uses less route discovery overhead than Source routing because both the source and destination end stations can simultaneously discover the route. Congestion and Load Balancing Source routing does not automatically reroute around congested or failed links or perform load balancing. Transparent bridging does not make use of idle standby paths during normal operation or offer load balancing in the event of congestion. SRT makes more efficient use of the available bandwidth by switching to standby paths in case of a failure on the primary link. Network Consolidation SRT-compliant devices support both 4M-bps and 16M-bps Token Ring Lans and can consolidate