McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Chapter 12 Connecting LANs and WANs: Making Backbone Networks

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Differentiate between the different connecting devices and the layers each supports. Understand the functionality of a repeater and where it is used. Understand the difference between a two-layer and a three- layer switch. After reading this chapter, the reader should be able to: O BJECTIVES Understand the functionality of a bridge and where it is used. Understand the functionality of a router and where it is used.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 O BJECTIVES (continued) Distinguish between a logical bus and logical star backbone. Distinguish between a bridge-based and a router-based back- bone network. Understand the functionality of a backbone network and where it is used.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 CONNECTINGDEVICESCONNECTINGDEVICES 12.1

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-1 Connecting devices

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-2 Repeater

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A repeater connects segments of a LAN together. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A repeater forwards every packet; it has no filtering capability. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Repeaters and Amplifiers It is tempting to compare a repeater to an amplifier, but the comparison is inaccurate. An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates it. When it receives a weakened or corrupted signal, it creates a copy bit for bit, at the original strength. A repeater is a regenerator, not an amplifier.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-3 Hubs

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Active versus Passive Hub A passive hub is a hub that is used to create a connection; it does not regenerate signals. An active hub, on the other hand, is a connector as well as a repeater. Today, most hubs are active hubs.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A bridge has a table used in filtering decisions. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-4 Bridge

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A bridge connects segments of a LAN together. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A router is a three-layer (physical, data link, and network) device. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A repeater or a bridge connects segments of LAN together A router connects independent LANs or WANs to create an internetwork (internet). Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-5 Routing example

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A router changes the physical addresses in a packet. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Multiprotocol Router At the network layer, a router by default is a single-protocol device. In other words, if two LANs are to be connected by a router, they must use the same protocol at the network layer. The reason behind this is that the routing table must use one single addressing format. However, multiprotocol routers have been designed to route packets belonging to two or more protocols. For example, a two-protocol router (for example, IP and IPX) can handle packets belonging to either of the two protocols. It can receive, process, and send a packet using the IP protocol or it can receive, process, and send a packet using the IPX protocol. In this case, the router has two routing tables: one for IP and one for IPX.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Brouters A brouter (bridge/router) is a single-protocol or multiprotocol router that sometimes acts as a router and sometimes as a bridge. When a single-protocol brouter receives a packet belonging to the protocol for which it is designed, it routes the packet based on the network layer address; otherwise, it acts as a bridge and passes the packet using the data link layer address. Likewise, when a multiprotocol brouter receives a packet belonging to one of the protocols for which it is designed, it routes the packet based on the network layer address; otherwise, it acts as a bridge and passes the packet using the data link layer address.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Gateways Today, the term gateway is used mostly as a synonym for a router. However, in the past, the two terms had different meanings. A gateway defined a device that could potentially operate in all five layers of the Internet model. It was a protocol converter. A router was a device that could transfer, accept, and relay packets only across networks using similar protocols. A gateway, on the other hand, could accept a packet formatted for one protocol (e.g., AppleTalk) and convert it to a packet formatted for another protocol (e.g., TCP/IP) before forwarding it.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 BACKBONENETWORKSBACKBONENETWORKS 12.2

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In a logical bus backbone, the logical topology of the backbone is a bus; the physical topology can be a bus or a star. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-6 Bridged-base logical bus network

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-7 Router-based logical bus backbone

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In a logical star backbone, the logical topology of the backbone is a star; the backbone is just one switch. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 12-8 Logical star backbone

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: FDDI Fiber distributed data interface (FDDI) is a local area network protocol standardized by ANSI and the ITU-T. It supports data rates of 100 Mbps. FDDI is implemented as a dual ring as shown below.