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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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This classification of devices is important for understanding data flow on a network and also for troubleshooting networks. Data flow through Layer 2 devices always involves a physical, hardware, or MAC address. Frames are processed created at this layer by NICs. Frames are processed at this layer by bridges and by multiport, microsegmented bridges called switches. Thus the student should be encouraged to classify NICs, bridges, and switches as related devices. From a troubleshooting point of view, any problems involved in framing the data or source or destination MAC address may be described as Layer 2 problems.</p> <p>Graphic one shows the OSI layers for one end node on the left, then the OSI layers for the networking device in the middle, and then the OSI layers for the other end node on the right. Note that for the Layer 2 networking device in the middle, the data packet is de-encapsulated up to the Layer 2 level and then re-encapsulated. The data packet is processed at Layers 1 and 2 only.</p> <p>This TI relates to CCNA Certification Exam Objective #1.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>It is important to remember that packets are contained inside frames. To understand how packets travel on Layer 2 devices, it is necessary to understand the frame, which encapsulates the packet. Anything that happens to the frame also happens to the packet.<img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121118/knet/v214adataimage1/1.gif" width="12" height="12"></p><p>Figures <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121118/knet/v214adataimage1/1.gif" width="12" height="12"> <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121118/knet/v214adataimage2/2.gif" width="12" height="12"> <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121118/knet/v214adataimage3/3.gif" width="12" height="12"> show that certain devices operate at Layers 1 and 2. NICs, bridges, and switches use of data link (MAC) address information to direct frames, which means they are Layer 2 devices. The MAC address, which is 'burned' permanently on the NIC, is the key information that is used to create the frame.</p><p>Bridges <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121118/knet/v214adataimage3/3.gif" width="12" height="12"> work by examining the MAC address of incoming frames. If the frame is local (with a MAC address on the same network segment as the incoming port of the bridge), then the frame is not forwarded across the bridge. If the frame is nonlocal (with a MAC address not on the same network segment as the incoming port of the bridge), then it is forwarded to the next network segment.</p><p>The details of switching will be covered in Semester 3. At this point, a switch may be considered to be a bridge with multiple ports, and with the appearance of a hub. The switch takes a data frame, reads the frame, examines the Layer 2 MAC addresses, and forwards (switches) the frame to the appropriate port.</p><table cellpadding="0" cellspacing="4" width="63%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td valign="middle" width="8%"><img height="23" width="23" src="../../../../../images/ccna/common/icon2.gif"></td><td valign="middle" width="92%"><span class="cstitle">Web Links</span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci212736,00.htm">Packets</a></span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.cs.mun.ca/~donald/bsc/node13.html">Data Encapsulation</a></span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp99/i02_fnd/sld006.htm">Encapsulation Slide</a></span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/encapsulation.html">Encapsulation of PDUs</a></span></td> </tr> </table></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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