Chapter 5: Managing Bridges, Routers, Gateways

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5.1 Introduction

LANs have become the primary means of moving data between computers and accessing applications and services. The largest companies may have hundreds of LANs among their locations worldwide—at the headquarters, research facilities, manufacturing plants, sales branches, and service centers—interconnecting tens of thousands of employees. Even many of their employees may have LANs at home for sharing computer peripherals, applications, and a broadband Internet connection. Depending on the specific situation, interconnectivity between locations is accomplished through the use of repeaters, bridges, routers, and gateways. Although the functionality of each is often embedded into a single device, the differences are best illustrated by discussing their relationship to the seven-layer OSI reference model.

• Layer 1 (Physical). In the digital environment of LANs, repeaters can be used to boost incoming signals for transmission over longer distances. But they also amplify noise. A better device is a regenerator, which reissues incoming 1s and 0s. On the output side, this eliminates noise from the transmission, ensuring consistent signal quality end-to-end. Repeaters and regenerators lack the intelligence to distinguish between frames, but they effectively increase the distances of Ethernet or token-ring networks. There is a limit on the number of repeaters and regenerators that can be used to extend the LAN before delay becomes a problem. But this can be overcome by using fiber-optic extenders, which are also Layer 1 devices.

• Layer 2 (Data Link). A bridge connects LANs at a relatively low level, the data link layer (DLL); specifically the LLC sublayer. The bridge moves traffic between LAN segments by filtering the source and destination addresses of the packets offered to it. Since it only reads addresses and looks no further into the packets, it does not care what protocol is used. This means that a bridge can interconnect DECnet, TCP/IP, or Xerox Network System (XNS) networks, for example, without being concerned about higher level protocols. These devices can also be used to bridge data between different media types, such as Ethernet and FDDI or token ring to Ethernet. Another DLL sublayer is the MAC, which regulates access to the LAN. All bridges attach to the LAN at this sublayer, just like a computer does via a NIC.

• Layer 3 (Network). A router selects the optimal path for packets offered to it based on such metrics as destination, QoS, and traffic type. Since the router looks deeper into the packets, it is protocol sensitive. For instance, a router could be used to connect a LAN running IP over Ethernet with a WAN running IP over frame relay.

• Layer 4 (Transport). The transport layer is an end-to-end process that complements Layer 3. If IP packets, for example, do not arrive at their destination or arrive as errored packets, TCP operating in the client device requests a retransmission. TCP also puts IP packets arriving out of sequence into the proper order before passing them to the higher layers.

• Layers 5–7 (Session, Presentation, and Application). A gateway employs protocol conversion and other translations to connect different types of networks, such as voice over IP (VoIP) to the public-switched telephone network (PSTN). In essence, the facilities of one network must be converted to the equivalent facilities of the second type of network. Because of the amount of processing that must be done, gateways are much slower compared to bridges and routers in terms of the common performance metric— packets per second (pps).

A specified protocol may be either routed or bridged on a given interface, but not both. In other words, while the same device can handle both bridging and routing functions, routed traffic is confined to the routed interfaces and bridged traffic is confined to bridged interfaces. Whether used for bridging or routing, or both, the device is configured by the vendor or IT staff using an extensive array of commands.

Repeaters, bridges, routers, and gateways are manageable with the SNMP or the element management system (EMS) supplied by the vendor. The difference is that SNMP uses a text-oriented command line interface to implement basic functions, whereas the EMS is a GUI that makes the SNMP functions easier to implement. Vendors may also provide their EMSs with proprietary features. Chapter 11 discusses network management in more detail.

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