im通用规则2：rpf接口是作为对源ip地址(或稀疏模式(*,G)项，rp)的最小开销路径接口(基于管理距离/cost)来估算的。如果多个接口
cost相同，选择最高ip地址的接口作为"切入点"（tiebreak）。
im通用规则3：在创建新的(S,G)项时，其输出接口列表提供一个来自父(*,G)项的输出接口列表的副本。
im通用规则4:组播转发项的输入接口(rpf接口),决不能在其他自己的输出接口列表上出现。
im通用规则5:每个组播状态项的rpf接口(即输入接口)每5秒钟重新刷新一次，而且依据规则4对输出接口的列表做适当的调整(以防输出接口列表出现输入接口).
im通用规则6:对(*,G)项的输出接口列表的添加和删除都被复制到所有与该组有关的(S,G)项。
dense模式规则
dense模式规则 1:密集模式(*,G)项的输出接口列表映射了现有的pim-dm邻居接口或直接连接的组成员接口。
dense模式规则 2:密集模式(S,G)项的输出接口在修剪时不能删除，而是被标记为"prune/dense"，且仍保留在输出接口列表上。
dense模式规则 3:当一个接口的pim邻居列表上增加一个新邻居时，该接口在所有pim-dm (S,G)输出列表重新设置"forward/dense"状态。
arse模式规则
arse模式规则 1:稀疏模式(*,G)项是作为显式加入操作结果而创建的。
arse模式规则 2：稀疏模式(*,G)项的入口总是指向rp的共享树。
arse模式规则 3：稀疏模式(S,G)项在下列条件下创建：
---接收到一条(S,G)加入消息/修剪消息。
---上一跳路由器切换到spt时。
---不可预料的(S,G)到达时(*,G)状态不存在。
---在rp上接收到一条注册(register)消息时
arse模式规则 4：增加到稀疏模式(*,G)或(S,G)项出口表中的接口有如下两个条件之一决定：
---当通过该接口接收到一个适当的(*,S)或(S,G)加入消息时。
---当该接口存在一个该组的直连成员时。
arse模式规则 5：从稀疏模式(*,G)或(S,G)向的出口表中删除一个接口发生如下两种之一的情形：
---当该接口(这里没有直连成员)接收到一个适当的(*,G)或(S,G)修剪消息(不被否决)时。
arse模式规则 6：接口有效期计时器重新置回3分钟是通过如下两个条件之一来进行的：
---该接口接收到一个适当的(*,G)或(S,G)加入消息。
---在接口上接收到一个来自直连成员的IGMP membership report
arse模式规则 7:当(S,G)项的rpf邻居与(*,G)项的rpf邻居不同时，路由器将向共享树发送一个(S,G)rp位修剪消息。
arse模式规则 8：稀疏模式(S,G)项的rpf接口(即入口)是用源的ip地址计算的，除了设置rp位的情况外，在这种情况，是用rp的ip地址。
is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. TheData transfer involves transmitting data between the devices over the virtual circuit, and the circuit terminationhase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.
is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtualcircuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
Dialup services offer cost-effective methods for connectivity across WANs. Two popular
DDRwhen it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period.
is another way of configuring DDR. However, in dial backup, the switchedcircuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching.The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.
WANs use numerous types of devices that are specific to WAN environments. WAN switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.
carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model. Figure 3-5 illustrates two routers at remote ends of a WAN that are connected by WAN switches.
acts as a concentration point for dial-in and dial-out connections. Figure
is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form. Figure 3-7 illustrates a simple modem-to-modem connection through a WAN.
connect a router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication between these devices. Figure 3-8 illustrates the placement of the CSU/DSU in a WAN implementation.
connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter ismodem, although it is called a terminal adapter because it does not actually convert analog to digital signals. Figure 3-9 illustrates the placement of the terminal adapter in an ISDN environment.
Point-to-point, packet-switched, and circuit-switched.
What is DDR, and how is it different from dial backup?
