Ref: 99980076 Title: Internetworking Technology Date: 12/1/87 Copyright 3Com Corporation, 1991. All rights reserved. Internetwork products play an important role in LAN technology because they allow LANs to be interconnected to extend their length, to combine two physical networks into one logical network, or to connect networks with different technologies. Internetworking products can be divided into four categories: Repeaters, Bridges, Routers, and Gateways. The following sections describe in more detail the functionality of these products. .h1;Repeaters Repeaters are used to extend the length of a single network. Repeaters are the least sophisticated of the internetworking products in that they operate at the physical layer. They receive all data and repeat or retransmit the data on the other segment. Their operating speeds are dependent on the local network speed. Repeaters are typically hardware devices and do not contain any network management functions. There are generally hardware limitations on the number of repeaters allowed in a system. For example, in a standard IEEE 802.3 Ethernet network, a maximum of four repeaters can be installed to form a network. .h1;Bridges Bridges operate at the second level of the OSI network architecture (Data Link Layer) and are totally transparent to network users. Bridges interconnect LANs of similar or dissimilar type to create an "Extended LAN." Because bridges are "protocol independent," protocols such as XNS, TCP/IP, DECNet, or ISO can simultaneously run on the extended LAN. Bridges provide much more than just protocol independence; they feature localized traffic, auto configuration, integrated network management, generalized filtering capability, and high- performance throughput. Bridges do not retransmit all traffic like a repeater. Bridges receive all packets on an attached network. They filter (discard) any packets whose destination is on that network and forward (regenerate) only the packets destined for the remote side. In this way, bridges route packets based strictly on the Destination Address in the data link frame; they do not depend on any routing protocol. Network devices on each side of a bridge simply address the packets to other devices as if they were on the same LAN; the process of forwarding packets to the remote LAN are transparent to the devices. Bridges do not depend on any other protocol beyond the link layer addresses (i.e., they are "protocol independent"). A smart bridge "learns" which devices are on which side by monitoring the Source Addresses of all packets. The bridge builds a routing table based on the packet's Source Address and the network from which it was received. Thus, whenever a device transmits a packet, its location is detected by the bridge and an entry is made in the routing table. This "learned" table is then used for the filtering and forwarding operations. Some bridges may not support "learning", in which case the table is configured manually by a network administrator. .h1;Routers Routers are also used in local and remote networks to form an extended LAN. They allow the user to build complex internets. Routers operate on the third OSI layer (Network Layer), which provides information for routing packets through an internetwork. Network devices with different protocols at layer three cannot communicate via routers. So, whereas devices of different architecture (XNS, TCP/IP, DECNet) may share the transmission capacity of a LAN, they may not be able to share the service provided by a router. The key advantage of routers is that the network layer protocol allows more sophisticated routing (e.g., alternate routing and load balancing between parallel paths). Routers require a different network ID between them to distinguish a local from a remote network. .h1;Gateways Gateways are used to interconnect networks with different protocol architectures. Gateways operate at the higher layers (above the network layer) of the OSI model. They translate protocols from one architecture to the other. Gateways are complex products and are required to perform substantial protocol processing. Examples of Gateways are XNS to SNA (Bridge Communications' CS/1-SNA) or XNS to X.25 (Bridge Communications' GS/1-X.25). .h1;OSI Model and Internetworking Products In the OSI model, the first layer is the Physical Layer, which defines the rules (such as electrical level and physical characteristics) for communicating across the actual transmission media. It also specifies the means of joining two able segments together by a repeater. A repeater merely accepts databits on one side and retransmits them to the other side. In the process, it enables the original signals to travel a longer distance. In the Ethernet specification, a single segment is limited to 500 meters. Using repeaters, Ethernet can be extended to 1,500 meters (using a maximum or two repeaters according to Ethernet version 1.0) or 2,800 meters (using a maximum of two repeaters according to Ethernet Version 1.0) or 2,800 meters (using a maximum of four repeaters, per Ethernet Version 2.0 or IEEE 802.3). However, the connected segments are one physical LAN and all traffic is present on each segment. OSI layer two is the Data Link Layer. In the data link layer, databits are grouped as packets. Each packet contains a Source Address and a Destination Address. The rules at this level define how these packets can access the physical media (i.e., packet type information and Logical Link Control information). Bridges operate at this layer to interconnect two physically distinct LANs to form an extended LAN. OSI layer three is the Network Layer. The network layer contains the rules defined for interconnecting networks (LANs are a subset of general networks) to form an internet. In particular, network addresses such as IP addresses and XNS network addresses are used to distinguish one LAN from another. Network routers join one LAN to another. LANs connectd by a router are physically and logically separated networks, whereas LANs connected by a bridge are physically different networks but logically the same network (they share the same network address). The network layer also contains the rules for passing routing information and making routing decisions (i.e., the requirements for internetworking and routing protocols such as XNS's IDP and RIP). Each network device wanting to use a router must participate in the same internet protocol and explicitly address the router to forward any internet packets. A router simply forwards any incoming packets without monitoring or filtering traffic. Routers can be used to build an internet with a complex network topology containing many LANs with multiple communication paths and loops. Each network or LAN within an internet can support its own network management functions without interfering with others. Thus, routers are used in different applications from bridges; just as bridges are used differently from repeaters. They provide interconnection of LANs at different levels and provide different functions and capabilities. COMPARISON AMONG REPEATERS, BRIDGES, ROUTERS, AND GATEWAYS Functions Repeater Bridge Router Gateway ------------------------------------------------------------------------------ OSI Layer Physical Data Link Network Above network Filter No Yes Not required Not required Capability (receives intenet traffic only) Forward Forward bits Forwards Forwards Forwards messages Capability data link internet packets packets Forwarding 10M bps 1000 pkts 100 pkts 10 to 100 pkts Performance or (network per second per second per second speed) Addressing No address MAC address Network address Network address Function Ethernet One physical One logical Multiple logical One physical LAN Extension LAN w/ max LAN w/ multi LAN w/ unlimited 2,800m Ethernets distance Configuration Linear Tree type Arbitrary Arbitrary topology (no loops) topology topology Network No Yes Yes Yes Management Transparent Yes Yes No No to Upper Layer Protocol Cost Low Medium Medium Medium $1,000-2,500 >$10,000 $5,000-10,000 $10,000-20,000 Applications Interconnects Localizes Complex internet Protocol Ethernet traffic, Conversion segments mixed media