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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; dash = contentID.indexOf('-'); sub1 = contentID.substring(0,dash); sub2 = contentID.substring(dash+1); numbackParams = "CHAPID=" + "knet-v214aCH47511" + "/RLOID=" + "knet-v214aRLO47828" + "/RIOID=" + "knet-v214aRIO124657"; ContentRef = "../../../../../"+ numbackParams + "/"+ sub1 +"/"+ sub2 +"/"; eval('parent.frames.content_frame.location.href="'+ContentRef+'content.html"'); eval('parent.frames.media_frame.location.href="'+ContentRef+'media.html"'); eval('parent.frames.navigation_frame.location.href="'+ContentRef+'contentresourcenav.html"'); top.currentSupport++; top.firstSupportPage = contentID; top.prevPage = contentID; top.contentResourceID = contentID; } function launchReference(rioID, rloID, chapterID) { contentID = "knet-v214aCV47051"; dash = contentID.indexOf('-'); sub1 = contentID.substring(0,dash); sub2 = contentID.substring(dash+1); numbackParams = "CHAPID=" + chapterID + "/RLOID=" + rloID + "/RIOID=" + rioID; ContentRef = "../../../../../"+ numbackParams + "/"+ sub1 +"/"+ sub2 +"/"; eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_crf.html"'); chapter = "knet-v214aCH47511"; ch_dash = chapter.indexOf('-'); ch_sub1 = chapter.substring(0,ch_dash); ch_sub2 = chapter.substring(ch_dash+1); params = "CHAPID=" + chapter + "/RLOID=" + "knet-v214aRLO47828" + "/RIOID=" + "knet-v214aRIO124657"; 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">11.5</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Routing Protocols</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">11.5.2</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Definition of routing protocol</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>Routing protocol is defined. To make the example concrete, the long-established and widely available routing protocol RIP is studied as a simple example of a routing protocol. RIP represents some of the major aspects of routing protocols, such as the use of a metric to make routing decisions (in RIPs case, hop count) and an update process (to ensure timely communication amongst the routers).</p> <p>Emphasize to students that without routing protocols to update each other about the state of the network topology, disruptions in that topology (which grow more likely as the internetwork grows larger) become fatal to packets (they become undeliverable for lack of a path) trying to traverse the network. Routing protocols, when properly running, assure the routers have a consistent and up-to-date way to decide how to choose the best path.</p> <p>This TI relates to CCNA Certification Exam Objectives #36 and #39.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Routers use routing protocols to exchange routing tables and to share routing information. Within a network, the most common protocol used to transfer routing information between routers, located on the same network, is <i>Routing Information Protocol (RIP)</i>. This <i>Interior Gateway Protocol (IGP)</i> calculates distances to a destination host in terms of how many <i>hops</i> (that is, how many routers) a packet must pass through. RIP enables routers to update their routing tables at programmable intervals, usually every 30 seconds. One disadvantage of routers that use RIP is that they are constantly connecting to neighboring routers to update their routing tables, thus creating large amounts of network traffic.</p><p>RIP allows routers to determine which path to use to send data. It does so by using a concept known as <i>distance-vector</i>. Whenever data goes through a router, and thus, through a new network number, this is considered to be equal to one hop. A path which has a <i>hop count</i> of four indicates that data traveling along that path would have to pass through four routers before reaching the final destination on the network. If there are multiple paths to a destination, the path with the least number of hops would be the path chosen by the router.</p><p>Because hop count is the only routing metric used by RIP, it does not necessarily select the fastest path to a destination. A metric is a measurement for making decisions. There are other routing protocols that use many other metrics besides hop count to find the best path for data to travel. Nevertheless, RIP remains very popular, and is still widely implemented. This may be due primarily to the fact that it was one of the easiest routing protocols to be implemented.</p><p>One other problem posed by the use of RIP is that sometimes a destination may be located too far away to be reachable. When using RIP, the maximum number of hops that data can be forwarded through is fifteen. The destination network is considered unreachable if it is more than fifteen router hops away.</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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