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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-v214aRLO47675" + "/RIOID=" + "knet-v214aRIO121637"; 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">7.4</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Layer 2 Devices</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">7.4.4</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Bridge Layer 2 operations</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>The purpose of this target indicator is to explain how a bridge works in more detail and to make plausible its classification as primarily a Layer 2 device. Have the students act out the bridges filtering properties in a kinesthetic activity. For example, use five students: one student in the middle playing the role of a bridge, two students on the left portraying hosts on one network segment, and two students on the right portraying hosts on a different network segment. Label the hosts with MAC addresses A, B, C, D. Have the bridge create a bridging table, which shows each bridge interface and the MAC addresses of the hosts that are accessible through that interface. Have the hosts send frames to each other and have the bridge person explain their filtering and forwarding decisions.</p> <p>Switching is an extremely important topic on the CCNA Certification Exam. This TI relates to CCNA Certification Exam Objectives #46 through #60.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Bridging occurs at the data link layer, which controls data flow, handles transmission errors, provides physical addressing, and manages access to the physical medium. Bridges provide these functions by using various link layer protocols that dictate specific flow control, error handling, addressing, and media access algorithms. Examples of popular data link layer protocols include Ethernet, Token Ring, and FDDI.</p><p>upper layer protocol transparency is a primary advantage of bridging. Bridges are not required to examine upper layer information because they operate at the data link layer or Layer 2 of the OSI model. Bridges filter network traffic by only looking at the MAC address, not protocols. It is not uncommon for a bridge to move protocols and other traffic between two or more network segments. Because bridges only look at MAC addresses, they can rapidly forward traffic representing any network layer protocol. To filter or selectively deliver network traffic, a bridge builds tables of all MAC addresses located on their directly connected network segments.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage1/1.gif" width="12" height="12"></p><p>When data comes along the network media, a bridge compares the destination MAC address carried by the data to MAC addresses contained in its tables. If the bridge determines that the destination MAC address of the data is from the same network segment as the source, it does not forward the data to other segments of the network.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage2/2.gif" width="12" height="12"> -<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage5/5.gif" width="12" height="12"> If the bridge determines that the destination MAC address of the data is not from the same network segment as the source, it forwards the data to the appropriate segment.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage6/6.gif" width="12" height="12"> -<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage9/9.gif" width="12" height="12"> By doing this, bridges can significantly reduce the amount of traffic between network segments by eliminating unnecessary traffic. View the Figures <img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage2/2.gif" width="12" height="12"> -<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage5/5.gif" width="12" height="12"> to see how bridges handle local network traffic. In contrast, view Figures <img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage6/6.gif" width="12" height="12"> -<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47675/RIOID=knet-v214aRIO121637/knet/v214adataimage9/9.gif" width="12" height="12"> to see how bridges handle nonlocal network traffic.</p><p>Bridges are internetworking devices that can be used to reduce large collision domains. Collision domains are areas where packets are likely to interfere with each other. They do this by dividing the network into smaller segments and reducing the amount of traffic that must be passed between the segments. As data is passed along the network on its way to a destination, it is picked up and examined by every device on the network including bridges. Bridges work best where traffic is low from one segment of a network to other segments. When traffic between network segments becomes heavy, bridges can become a bottleneck and slow down communication.</p><p>There is another potential problem with using a bridge. Bridges always spread and multiply a special kind of data packet. These data packets occur when a device on a network wants to reach another device on the network, but does not know the destination address of the device. When this occurs, the source sends out a <i>broadcast</i> to all devices on a network. Since every device on the network has to pay attention to such broadcasts, bridges always forward them. If too many broadcasts are sent out over the network a broadcast storm can result. A broadcast storm can cause network time-outs, traffic slowdowns, and the network to operate at less than acceptable performance.</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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