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<html> <head> <meta content="en-us" http-equiv="Content-Language"> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../../css/ccna.css"> <script src="../../../../../js/framesetHelper.js" language="JavaScript"></script><script language="JavaScript"> window.scrollTo(0,0); function newPage(pageID) { alert("COMMING SOON!!\nThis feature is currently in development."); } function launchMoreInfo(oid) { dash = oid.indexOf('-'); sub1 = oid.substring(0,dash); sub2 = oid.substring(dash+1); // this is an URL base on the // system that generated the static // download of the curriculum. httpref = "http://nomar.sdf.cisco.com/servlet/org.cli.delivery.rendering.servlet.MIServlet/Static=true,LMSID=DTUI,Engine=dynamic,Theme=cnamstheme,Style=ccna,Language=en,Version=2,RootID=knet-v214aCV47051/CHAPID=null/RLOID=null/RIOID=null/"; newref = httpref + sub1 +"/"+ sub2 +"/moreinfoframeset.html"; //alert("new newref :\n" + newref); window.open(newref,'MoreInfo','toolbar=no,status=yes,menubar=no,location=no,scrollbars=yes,height=442,width=756,left=20,top=20,resizable=yes') } function launchContentResource(contentID) { top.currentSupport = 0; 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eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_crf.html"'); chapter = "knet-v214aCH47507"; ch_dash = chapter.indexOf('-'); ch_sub1 = chapter.substring(0,ch_dash); ch_sub2 = chapter.substring(ch_dash+1); params = "CHAPID=" + chapter + "/RLOID=" + "knet-v214aRLO47676" + "/RIOID=" + "knet-v214aRIO121640"; returnRef = "../../../../../"+ params + "/"+ ch_sub1 +"/"+ ch_sub2 +"/pageframeset.html"; top.prevPage = returnRef; } function launchFile(filepath) { newWindow = window.open(filepath,"","top=70,left=120,width="+(screen.width*.50)+",height="+(screen.height*.50)+",resizable=yes,scrollbars=yes,toolbar=yes,menubar=yes,status=yes"); /* if (is.ie && filepath.lastIndexOf(".pdf") != -1) { pageTimer = setInterval("winLoadCheck()", 1000); } */ } function winLoadCheck() { if (newWindow.document.readyState == 'complete') { clearInterval(pageTimer); newWindow.location.reload(); } } // BrowserCheck Object // provides most commonly needed browser checking variables // 19990326 // Copyright (C) 1999 Dan Steinman // Distributed under the terms of the GNU Library General Public License // Available at http://www.dansteinman.com/dynapi/ function BrowserCheck() { var b = navigator.appName if (b=="Netscape") this.b = "ns" else if (b=="Microsoft Internet Explorer") this.b = "ie" else this.b = b this.v = parseInt(navigator.appVersion) this.ns = (this.b=="ns" && this.v>=4) this.ns4 = (this.b=="ns" && this.v==4) this.ns5 = (this.b=="ns" && this.v==5) this.ie = (this.b=="ie" && this.v>=4) this.ie4 = (navigator.userAgent.indexOf('MSIE 4')>0) this.ie5 = (navigator.appVersion.indexOf('MSIE 5.0')>0) this.ie55 = (navigator.appVersion.indexOf('MSIE 5.5')>0) if (this.ie5) this.v = 5 this.min = (this.ns||this.ie) } // automatically create the "is" object is = new BrowserCheck()</script> </head> <body onLoad="window.focus();" alink="#000000" vlink="#000000" link="#000000" bgcolor="#E7E7E7" background="../../../../../images/ccna/common/bg.gif"> <table width="100%" cellspacing="0" cellpadding="2" border="0"><tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="rlohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="rlohdr">7.5</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Effects of Layer 2 Devices on Data Flow</td> </tr> </table> </td> </tr> <tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="riohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="riohdr">7.5.1</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Ethernet LAN segmentation</td> </tr> </table> </td> </tr> <tr> <td colspan="4"> <table width="90%" cellspacing="0" cellpadding="0" border="0"> <tr> <td class="smtext"> <p> </p><table cellpadding="0" cellspacing="0" width="100%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td width="100%"><img height="10" width="1" border="0" src="../../../../../images/ccna/common/transdot.gif"></td> </tr> <tr> <td width="100%"> <table bgcolor="#000000" bordercolor="#000000" cellspacing="0" bordercolorlight="#000000" width="100%" border="0"> <tr> <td width="100%"> <table bgcolor="#FFFFFF" cellspacing="0" cellpadding="2" width="100%" border="0"> <tr> <td valign="middle"><img height="22" width="22" border="0" src="../../../../../images/ccna/common/inotes.gif"></td><td valign="middle"><span class="cstitle">Instructor Note</span></td> </tr> <tr> <td valign="top"> </td><td valign="top"><span class="cstext"> <p>A crucial concept in understanding how real networks are designed is segmentation. Segmentation is also an important concept on the CCNA exam. This introduction to segmentation presents three devices for segmenting a network. These three devices are the bridge, the multiport bridging device (or switch), and the router. Note that repeaters, hubs, transceivers, and connectors, all being Layer 1 devices, cannot provide segmentation.</p> <p>This TI relates to CCNA Certification Exam Objective #46.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>There are two primary reasons for segmenting a LAN. The first is to isolate traffic between segments. The second is to achieve more bandwidth per user by creating smaller collision domains.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47676/RIOID=knet-v214aRIO121640/knet/v214adataimage1/1.gif" width="12" height="12"> Without LAN segmentation, LANs larger than a small workgroup would quickly become clogged with traffic and collisions, and would deliver severely reduced bandwidth. The addition of devices like bridges, switches, and routers segment the LAN in figure<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47676/RIOID=knet-v214aRIO121640/knet/v214adataimage2/2.gif" width="12" height="12"> into four collision domains.</p><p>By dividing large networks into self-contained units, bridges and switches provide several advantages.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47676/RIOID=knet-v214aRIO121640/knet/v214adataimage3/3.gif" width="12" height="12"> A bridge, or switch, diminishes the traffic experienced by devices on all connected segments, because only a certain percentage of traffic is forwarded. Both devices act as a <i>firewall</i> to stop some potentially damaging network errors. They also accommodate communication between a larger number of devices than would be supported on any single LAN connected to the bridge. Bridges and switches extend the effective length of a LAN, permitting the attachment of distant stations that were not previously permitted.</p><p>Although bridges and switches share most relevant attributes, several distinctions still exist between them. Switches are significantly faster because they switch in hardware, while bridges switch in software. Switches can interconnect LANs of unlike bandwidth. A 10 Mbps Ethernet LAN and a 100 Mbps Ethernet LAN can be connected by using a switch. Switches can support higher port densities than bridges. Some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store and forward traffic switching. Finally, switches reduce collisions and increase bandwidth on network segments because they provide dedicated bandwidth to each network segment.</p><p>Segmentation by routers has all of these advantages and more. Each interface on the router connects to a separate network, so insertion of the router into a LAN creates smaller collision domains and smaller broadcast domains. This occurs because routers do not forward broadcasts unless programmed to do so. However, the router can perform bridging and switching functions. The router can perform best path selection. The router can be used to connect different networking media, and different LAN technologies.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47676/RIOID=knet-v214aRIO121640/knet/v214adataimage4/4.gif" width="12" height="12"> Note that the router in the figure is connecting Ethernet, Token Ring and FDDI LAN technologies. It is segmenting the LAN, but doing much more. Routers can connect LANs running different protocols (IP versus IPX versus AppleTalk) and can have serial connections to WANs.</p></td> </tr> </table> </td> </tr></table> <p> </p> <table width="100%" cellspacing="0" cellpadding="2" border="0" valign="bottom"> <tr> <td></td> </tr> </table> </body></html>⌨️ 快捷键说明
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