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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 3 devices always involves hardware and a logical (IP) address. Packets are processed and created at this layer and then processed by routers. From a troubleshooting point of view, any problems involved in packets or source and destination IP addresses may be described as Layer 3 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 3 networking device in the middle, the data packet is de-encapsulated up to the Layer 3 level and then re-encapsulated. The data packet is processed at Layers 1 and 2 and 3.</p> <p>This TI relates to CCNA Certification Exam Objective #1.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>The main device that is discussed at the network layer is the router. Routers actually operate at Layer 1 (bits on the medium at router interfaces), Layer 2 (frames switched from one interface to another), based on packet information and Layer 3 (routing decisions).<img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121119/knet/v214adataimage1/1.gif" width="12" height="12"></p><p>Packet flow through routers <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121119/knet/v214adataimage2/2.gif" width="12" height="12"> (for the selection of best path and actual switching to the proper output port) involves the use of Layer 3 network addresses. After the proper port has been selected, the router encapsulates the packet in a frame again to send the packet to its next destination. This process happens for every router in the path from the source host to the destination host.</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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