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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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The need for more distance between computers led to the development of the Layer 1 device repeater (a concept borrowed from other telecommunications technologies). The need for more workgroup connectivity, along with the benefits of repeating (amplification/regeneration and retiming), led to the multiport repeater, or hub. Both repeaters and hubs, as Layer 1 devices, do not examine the information that passes through them. They simply deal with individual bits in the bit stream, regenerating and retiming these signals. The limitations of the hub (the fact that it does not filter network traffic at all) became apparent as more PCs were added to hubs and began to share bandwidth.</p> <p>The bridge was introduced as a way to filter network traffic into local and nonlocal traffic. This filtering was accomplished by physical layer addresses, thus making it a Layer 2 device. Bridges were introduced to segment networks into smaller collision domains. The basic idea of bridges was added to the connectivity (port-density) of hubs and the switch (a multiport bridge) was born. Also, a Layer 2 device that makes forwarding decisions based on the MAC addresses, the switch, provides high port density (connectivity) and dedicated bandwidth between two communicating PCs. As networks grew, the diversity of platforms, protocols, and media, the geographic distance between computers, the number of computers wishing to communicate, and the dynamism inherent in large networks all necessitated the development of the router. The router is a Layer 3 device, which makes best path and switching decisions based on Layer 3, hierarchical, network addresses.</p> <p>This TI relates to CCNA Certification Exam Objective #1.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Hosts and servers operate at Layers 1-7. They perform the encapsulation process. Transceivers, repeaters, and hubs are all considered active Layer 1 devices, because they act only on bits and require energy. Patch cables, patch panels, and other interconnection components are considered passive Layer 1 components because they simply provide some sort of conducting path.<img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47535/RIOID=knet-v214aRIO121115/knet/v214adataimage1/1.gif" width="12" height="12"></p><p>NICs are considered Layer 2 devices since they are the location of the MAC address. However, since they often handle signaling and encoding they are also Layer 1 devices. Bridges and switches are considered Layer 2 devices because they use Layer 2 (MAC address) information to make decisions on whether or not to forward frames. They also operate on Layer 1 in order to allow bits to interact with the media.<img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47535/RIOID=knet-v214aRIO121115/knet/v214adataimage2/2.gif" width="12" height="12"></p><p>Routers are considered Layer 3 devices because they use Layer 3 (network) addresses to choose best paths and to switch packets to the proper route. Router interfaces operate at Layers 2 and 1 as well as Layer 3. Clouds, which may include routers, switches, servers, and many devices we have not yet introduced, involve Layers 1-7.</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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