看下图,Video Server 以组播形式向外发送组播信息,根据前面的介绍我们知道,收到 路由 器的 CGMP 消息的交换机会将多播数据无条件的向支持 CGMP 协议 的 路由 器转发,因为我们应该尽量的让 多播源靠近 路由 器,以减少对网络带宽的占用。 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q5293A961.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 连接远程网络的 Router-D是一台性能较低的设备,如果我们把它也同样接在核心交换机 Cat5000 上,大量的多播视频信息会被传递至 Router-D,尽管远程网络中没有任何的组播信息接收者。图中的 连接方法是合适的,如果远程网络上没有多播信息的接收者,Router-A 到 Router-D 链路上的多播传 递会被修剪掉。 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q53000G60.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 可是上面的图形中仍有问题没有解决, Router B 和 Router C 都收到了一组它们并不需要的多 播 信息,因为 路由 器之间靠 PIM 而不是 IGMP 协议 来传递多播转发信息,第二层的交换机不知晓PIM 包 的含义,只能假定 路由 器需要所有多播组的数据流,为了让核心交换机能够 在 路由 器之间准确转发多播信息,我们必须增加第二层上的多播组转发信息通告方法。 23.6.1 DVMRP 邻居建立 DVMRP探针(Probe)消息定期向所有DVMRP 路由 器组地址(224.0.0.4)组播 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q5305A0P.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 1. 路由 器发送探针信息包,此时R1没有受到任何来自其他 路由 器的探针,邻居表为NULL 2. R2收到R1发送的探针,并向接口上的DVMRP邻居内部列表中加入R1的地址 3. R2在邻居列表上发送带有R1 ip的探针 4. R1收到探针,并将R2加入邻居列表,同时发送带有R2 ip的探针 此时双向邻接的工作已经完成,同时每隔10s发送一个,维持邻居关系,35s后未收到Probe包,则 宣告邻居死亡。DVMRPv3通过IGMPv2查询来决定指定 路由 器,指定 路由 器是唯一能发送多播会话响应 子网中IGMP查询的 路由 器 23.6.2 DVMRP 消息格式 1. Probe消息 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q53125U93.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q531563331.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q532192134.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q5324T409.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 23.6.3 DVMRP 路由 表 DVMRP采用了RIP的很多变量来对外告知 路由 表和直连子网, 路由 更新每60s发送一次,如果140s 未更新,则删除 路由 。Metric被设置为1~63,其中1~31为可达源,33~63为依赖源 Metric定义类似于RIP,每经过一个 路由 器 1 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q5332N5Y.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 路由 器发送一条毒性逆转的 路由 条目,即在Metric上加上无限(32),则上游 路由 将其认为依赖 路由 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q53355GZ.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 23.6.4 DVMRP 源树 路由 器A,B都向 路由 器 路由 器C,D向网络S通告一条Metric=1的指向S的 路由 。由于D对于S来说位 于 路由 器B的下游,因此为了向网络S逆向抑制通告 路由 ,并向B返回通告,将其 路由 变为依赖 路由 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q5344bB0.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 23.6.5 DVMRP组播转发 由于组播 路由 是一种颠倒的 路由 ,因此在DVMRP 路由 表中的信息被用于确定是否在正确的接口收 到一个输入的组播信息包。否则,为了防止组播循环将放弃该信息包,基于输入接口的转发再次被当 作逆向路径转发(RPF),并且把为了确定信息包到达正确的接口所进行的测试称为RPF检查。 一个组播信息包到达错误的接口的RPF检查如下 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q536449393.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 本例中,一个来自源151.10.3.21的组播信息包通过接口s0抵达。但是,依据DVMRP 路由 表中的 151.10.0.0/16,来自源的组播信息包必须通过接口E1(不是S0)到达。于是,RPF检查失败,而且组播信 息包被悄悄丢弃。 注意:通常情况下,DVMRP操作稳定时.信息包不会抵达错误的接口,因为上游 路由 器不会转发 信息包,除非下游 路由 器逆向抑制第一个位置的 路由 .然而若网络拓扑刚刚改变,且DVMRP 路由 选择尚 未在所有 路由 嚣上收敛(注意,这是基于 路由 选择的距离向量),并有突如其来的DVMRP操作发生时, 上述差错就会出现 23.6.6 DVMRP裁剪 即使在分布树上没有正在使用的接收站点,本过程也会发生。和大多数密集模式一样.DVMRP 使用扩散-剪枝机制向网络中的所有 路由 器开始发送组播信息。就DVMRP来说,信息沿着截断的广播树 向下被扩散到任一存在的接收站点。 但是.为了保存宝贵的网络资源.你必须沿着截断广播树分支向下切断(或修剪),通问没有 接收站点的信息流;因此,没有任何直接相连的接收站点的叶 路由 器为了停止不需要的组播信息流及 剪枝截断广播树的不需要的分支,而向战新广播树上发送DVMRP信息。在DVMRP剪枝后仍然留下的是 对特定源的”十分布树或5PT”。 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q53J194R.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 不幸的是.由于DVMRP足扩散—剪枝 协议 ,因此.只要剪枝超时。由DVMRP产生的源分布树就恢 复截断广墦树(一般地,DVMRP剪枝只有两分钟的有效时间.2分钟之后它们终止,信息再次扩散)和 PIM-DM一样,这种扩散-剪枝方式也能引起在网络的DVMRP 路由 器里建立(S,G)状态。即使只有很少的 接收站点 注意:由于被网络中所有 路由 器的DVMRP 路由 表项的内容所描述.因此截断广播树本身作为 DVMRP剪枝的结果不会被剪枝;这些DVMRP 路由 表项不会被剪枝信息修改-相反地,在DVMRP剪枝消息 中(通常作为某个具体的(S,G)信息流的剪枝)收到的信息被存储在 路由 器中的某个独立数据结构中;此 信息用于修改在截断广播树下面的(S,G)信息流; screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q53SW011.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 23.6.7 DVMRP嫁接 DVMRP提供一种可靠的嫁接机制,此机制把先前面已经剪枝掉的分支嫁接回来。如果没有这一机 制,组中新主机的加入等待时间可能受到严重影响。因为在组播信息开始流动之前.上游 路由 器的剪 枝状态必须结束.由于基于沿着被剪枝的分支的 路由 器数和使用的超时性.所以,在主机开始收到组 播之前许多时间已经逝去。通过使用嫁接机制.DVMRP把加入延迟减少列几毫秒。 与剪枝机制的不可靠下—样,嫁接机制由于使用嫁接确认消息而变得可靠-这些消息由上游 路由 器返回以作为收到的嫁接消息。这一步骤防止由十阻塞而引起的嫁接消息丢失,阻塞会导致嫁接 过程失败: 在 路由 器Y上的接收站点2加入组播组之后紧接着由于 路由 Y仍然存在(Si,G)状态,其表示 路由 器被剪枝,因此它知道为了重建信息流,必须向上游 路由 器E发出嫁接消息 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q5392161T.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 23.6.8 DVMRP区域扩充 screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor='hand'; this.alt='Click here to open new window\nCTRL Mouse wheel to zoom in/out';}" onclick="if(!this.resized) {return true;} else {window.open('http://file:///C:/Documents and Settings/Eagles/My Documents/缁勬挱婧愭斁缃 拰璺濈 鍚戦噺缁勬挱璺 敱閫夋嫨鍗忚 - 涓 浗涓绘祦IT鎬濈 鍗庝负3COM寰 蒋Juniper璁よ瘉缃戠粶鎶 鏈 笓涓氱珯-鏃犲厔寮熶笉鎶 鏈 56Cto_Com).files/2Q539549335.jpg');}" onmousewheel="return imgzoom(this);" alt="" /> 由于DVMRP仅支持31跳,所以为了扩展DVMRP,可以使用一些其他措施,例如使用tunnel |
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