📄 ch3.htm
字号:
<HTML><HEAD><TITLE>Chapter 3 -- How Bridges Work</TITLE><META></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910"><H1><FONT SIZE=6 COLOR=#FF0000>Chapter 3</FONT></H1><H1><FONT SIZE=6 COLOR=#FF0000>How Bridges Work</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#HowBridgesWork">How Bridges Work</A></UL><HR><P>An intranet-especially one in a large corporation-can be a remarkablycomplex endeavor, consisting of many networks, including departmentallocal area networks, as well as larger <I>subnetworks</I>, whicheach are in essence collections of networks.<P>Bridges connect networks. When bridges were first developed theycould not connect incompatible networking architectures. Earlybridges functioned at the media-access control (MAC) section ofthe data-link layer of the OSI model (discussed in <A HREF="ch2.htm" >Chapter 2</A>.Ethernet and Token Ring architecture differ at the MAC sectionof the data-link layer, hence their incompatibility. Newer bridgesfunction at the logical-link control (LLC) portion of the data-linklayer. As long as the communication protocol on both networksis the same (IPX to IPX, for example), Ethernet and Token Ringnetworks can be linked by bridges. Connecting divergent networksis an essential requirement for intranets.<P>Bridges are also used to connect networks indirectly by long-distance,usually leased, lines. Remote bridges on Ethernet networks usea <I>transparent</I> routing technique to handle traffic. It isthe destination address of each packet that is read to determinethe action to be taken. If the destination is on the same network,bridges ignore the packet; bridges pass only packets that needto go to other networks. Token Ring operates somewhat differentlyin that it uses <I>source </I>routing. In this case, the bridgeuses a test message to calculate the best path between the sourceand the destination.<P>In order to determine whether to pass or drop a packet, bridgesrefer to a table, which is basically a list of addresses. Thesetables originally had to be built manually, causing considerablemaintenance every time a replacement or additional NIC addresswas put on the network. This led to the development of <I>learning</I>bridges, that is, bridges with software algorithms that couldbuild these tables automatically. Today's bridges build and maintaintheir tables by listening to cable traffic and checking packetsfor source addresses.<P>Bridges are relatively simple to set up and maintain, and theycan operate fairly fast since they simply decide whether to passor drop the packet. In the next chapter a similar but more complexpiece of equipment, the router, is discussed. Bridges are oftenused to segment and reduce local network traffic. Routers areusually used for connecting entire networks and subnetworks together.<P>Bridges and routers are sometimes combined into a single productcalled a <I>brouter</I>. A brouter combines the functions of bothbridge and router. It examines the outermost address to see ifthe data needs to be sent to another LAN over a bridge, and deliversinformation that way if possible. If, instead, it needs to besent using IP technology, it will act as a router does.<H2><A NAME="HowBridgesWork"><FONT SIZE=5 COLOR=#FF0000>How Bridges Work</FONT></A></H2><P>Bridges are hardware and software combinations that connect differentparts of a single network, such as different sections of an intranet.They connect local area networks (LANs) to each other. They aregenerally not used, however, for connecting entire networks toeach other, for example, for connecting an intranet to the Internet,or an intranet to an intranet, or to connect an entire subnetworkto an entire subnetwork. To do that, more sophisticated piecesof technology called routers are used. (See <A HREF="ch4.htm" >Chapter 4</A>for moreinformation on how routers work.)<OL><LI>When there is a great amount of traffic on an Ethernet localarea network, packets can <I>collide</I> with one another, reducingthe efficiency of the network, and slowing down network traffic.Packets can collide because so much of the traffic is routed amongall the workstations on the network.<LI>In order to cut down on the collision rate, a single LAN canbe subdivided into two or more LANs. For example, a single LANcan be subdivided into several departmental LANs. Most of thetraffic in each departmental LAN stays within the department LAN,and so it needn't travel through all the workstations on all theLANs on the network. In this way, collisions are reduced. Bridgesare used to link the LANs. The only traffic that needs to travelacross bridges is traffic bound for another LAN. Any traffic withinthe LAN need not travel across a bridge.<LI>Each packet of data on an intranet has more information init than just the IP information. It also includes addressing informationrequired for other underlying network architecture, such as foran Ethernet network. Bridges look at this outer network addressinginformation and deliver the packet to the proper address on aLAN<LI>Bridges consult a <I>learning </I>table that has the addressesof all the network nodes in it. If a bridge finds that a packetbelongs on its own LAN, it keeps the packet inside the LAN. Ifit finds that the workstation is on another LAN, it forwards thepacket. The bridge constantly updates the l<BR>earning table as it monitors and routes traffic.<LI>Bridges can connect LANs in a variety of different ways. Theycan connect LANs using serial connections over traditional phonelines and modems, over ISDN lines, and over direct cable connections.CSU/DSU units are used to connect bridges to telephone lines forremote connectivity.<LI>Bridges and routers are sometimes combined into a single productcalled a <I>brouter.</I> A brouter handles both bridging and routingtasks. If the data needs to be sent only to another LAN on thenetwork or subnetwork, it will act only as a bridge deliveringthe data based on the Ethernet address. If the destination isanother network entirely, it will act as a router, examining theIP packets and routing the data based on the IP address.</OL><HR><CENTER><P><A HREF="ch2.htm"><IMG SRC="PC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="#CONTENTS"><IMG SRC="CC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="contents.htm"><IMG SRC="HB.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="ch4.htm"><IMG SRC="NC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><HR WIDTH="100%"></P></CENTER></BODY></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -