缩写：MD，组播域；mSRC，组播源；mDST，组播目的地；RP，汇聚点；intf，端口；RT，路由器；USRT，上游路由器；DSRT，下游路由器
RPF：PIM里最关键的技术。由于组播没有可供RT收集的目的地信息，也就是说组播域内的RT没有路由收敛的概念，因此RT必须通过一定手段防止环路。RPF就是告诉RT，哪个端口才应该接收组播包。若不是从该端口收到的组播包，则判断其经过了环路，应该将其丢弃。RPF地址及端口与路由表项一致；
PIM：使用Hello建立邻居关系；使用PIM-Prune请求USRT停止转发组播，使用PIM-Join请求USRT转发组播
PIM-DM
使用PIM-Prune通告邻居，不需要相应的multicast流量。密集模式下，针对不同的（源，组播地址）生成不同的SPT。源树是由mSRC驱动的，即mSRC发送组播数据包后，所有收到该组播包的PIM-DM RT都将根据该数据包的（S，G）计算RPF，生成相应的SPT。这时，所有PIM-DM的RT，除了RPF intf以外，都转发该RPF，若遇到非RPF intf接收到组播包的情况，则向该端口发送PIM-Prune主动终断该部分的连接。而若在3min内RT所有PIM-DM模式的non-RPF intf为以下两种情况之一：
一、没有收到IGMP Report（IGMP v1）或收到IGMP Leave（IGMP v2）；
二、收到PIM-Prune；
则认为本RT不需要组播包，并向RPF intf发送PIM-Prune要求停止发送（S，G）的组播。RT接收到PIM-Prune的端口，若为广播多路访问端口，将等待3sec，目的是为了等待其它同一广播域的RT发送Join，此称为prune override。若3s过后没有PIM-Join，则将其状态置为Dense/Prune，停止转发组播，并设置3min的定时器，若定时器时间内收到PIM-Prune，则重置定时器；否则重新将该端口置为Dense/Forward，继续转发组播。
若intf收到DSRT发送的PIM-Graft，则表明需要立即恢复接收组播，若该端口被置为Dense/Prune，则重置为Dense/Forward并转发。
PIM-SM
使用PIM-join通告邻居，需要连接到RP上接收所有的（或指定的）流量。稀疏模式下，默认所有组播地址都使用同一棵根树（RPT）。根树是由mDST驱动的，即mDST发送组播请求后，以指向RP的RPF为上游方向发送PIM-Join，最终到达RP，从而建立一棵完整的RPT，。当且仅当RP收到至少一个IGMP-Report或者PIM-Join后，才开始转发mSRC发送到的组播包。
注意有三步：1、DSRT使用PIM-Join连接到RP；2、RP接受mSRC所发送的PIM-Reg单播包，并通过RPT组播到mDST；3、RP发送PIM-Join到mSRC，建立SPT，之后mSRC到RP的数据流就使用组播发送；4、mDST获取组播包，了解了mSRC后，可主动发送PIM-Join到mSRC，建立SPT，并往USRT发送PIM-Prune删除RPT。
PIM-SDM：由于PIM-SM在使用auto-RP时，前期不知道RP的位置。为确保能接收到224.0.1.40（RP Discovery）的组播包，首先使用PIM-DM进行组播包接收，待收到RP Discovery之后，确定了RP位置，再转回PIM-SM模式。这就是PIM-SDM。
关联RP的方式 一、静态指定
在全局配置模式下输入：ip pim rp-address rp-address [acl] [override]
二、Auto-RP
使用Auto-RP时，RP通过224.0.1.39向所有agent发送rp-announce，agent收到这些rp-announce后，选择最优RP，并通过224.0.1.40向所有PIM-SM成员发送rp-discovery。注意：由于224.0.1.39、224.0.1.40也是两个不同的组播域，要接收这些组播信息，必须生成相对应的SPT，这就是为何需要PIM-SDM的原因了。
三、BootStrap Router（BSR）
candidate-BSR往224.0.0.13发送bootstrap msg，其它c-BSR收到后共同选举出一个BSR。而candidate-RP收到这些msg后，以unicast形式发送c-rp-adv给所选定的BSR。BSR在通过224.0.0.13向所有PIM RT通告所有的c-RP。PIM RT自己判断使用哪个RP。
四、MSDP
无论使用Auto-RP或者BSR，只要所有的RP使用同一个IP地址，即可实现同组Load-Balance。但如何使这些RP为根的RPT之间转发组播呢？答案是MSDP。运行MSTP后，RP之间会交换所有已知mSRC的IP，其余RP将与该源建立SPT，最后实现LB。
路由条目
SPT：源树，以（S，G）节点生成图。其中RPF指向mSRC，incoming intf为指向mSRC的端口，其它intf均为outgoing intf。若incoming intf为null，且RPF为0.0.0.0，则本RT直连mSRC；
RPT：根树，以（*，G）节点生成图。其中RPF指向RP，incoming intf为指向RP的端口，若有端口收到PIM-join，则置该端口为outgong intf。若incoming intf为null，且RPF为0.0.0.0，则该RT直连RP。
flags：
D：以PIM-DM模式转发；
S：以PIM-SM模式转发；
P：该条目已经被Prune；
C：DST直连，指该RT为收到IGMP report的路由器；
J：（仅用于PIM-SM）RPT-〉SPT，指通过本RT的multicast已从RPT转为SPT进行转发。当该条目用于（*，G）里面，代表SPT-Threshold已经超过预定值（Cisco默认1min0字节），下一个包将引发建立SPT；当条目用于（S，G）里面，表示SPT-Threshold已经低于预定值，将删除SPT重新使用RPT；
T：SPT，使用SPT进行转发；
L：RT本身就是组播成员；
F：（没搞懂）
由于配置基本一样，只贴出R2（RP）、R3、R8的配置：
RP：
hostname R2
ip cef
ip multicast-routing
!
interface Loopback0
ip address 2.2.2.2 255.255.255.255
ip pim sparse-dense-mode
!
interface FastEthernet0/0
ip address 172.16.12.2 255.255.255.0
ip pim sparse-dense-mode
!
interface FastEthernet1/4
no switchport
ip address 172.16.24.2 255.255.255.0
ip pim sparse-dense-mode
!
interface FastEthernet1/5
no switchport
ip address 172.16.25.2 255.255.255.0
ip pim sparse-dense-mode
!
interface FastEthernet1/7
no switchport
ip address 172.16.27.2 255.255.255.0
ip pim sparse-dense-mode
!
interface FastEthernet1/8
no switchport
ip address 172.16.28.2 255.255.255.0
ip pim sparse-dense-mode
!
router ospf 100
router-id 2.2.2.2
log-adjacency-changes
passive-interface Loopback0
network 2.2.2.2 0.0.0.0 area 0
network 172.16.12.2 0.0.0.0 area 0
network 172.16.24.2 0.0.0.0 area 0
network 172.16.25.2 0.0.0.0 area 0
network 172.16.27.2 0.0.0.0 area 0
network 172.16.28.2 0.0.0.0 area 0
!
ip pim send-rp-announce Loopback0 scope 2
ip pim send-rp-discovery Loopback0 scope 2
ip pim rp-candidate Loopback0 priority 5
!
end
------------------------------------------------
Source：
hostname R3
ip cef
ip multicast-routing
!
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface Loopback1
ip address 3.0.0.1 255.255.255.0
ip ospf network point-to-point
!
interface FastEthernet0/0
ip address 172.16.13.3 255.255.255.0
ip pim sparse-dense-mode
!
interface Serial0/0
ip address 172.16.34.3 255.255.255.0
ip pim sparse-dense-mode
!
interface Serial0/1
ip address 172.16.36.3 255.255.255.0
ip pim sparse-dense-mode
!
router ospf 100
router-id 3.3.3.3
log-adjacency-changes
passive-interface Loopback0
passive-interface Loopback1
network 3.0.0.1 0.0.0.0 area 345
network 3.3.3.3 0.0.0.0 area 345
network 172.16.13.3 0.0.0.0 area 0
network 172.16.34.3 0.0.0.0 area 345
network 172.16.36.3 0.0.0.0 area 36
!
end
------------------------------------------------
Destination：
hostname R8
ip cef
ip multicast-routing
!
interface Loopback0
ip address 8.8.8.8 255.255.255.255
!
interface Loopback1
ip address 8.0.0.1 255.255.255.0
ip pim sparse-dense-mode
ip igmp join-group 224.8.8.8
ip ospf network point-to-point
!
interface Serial0/0
ip address 172.16.58.8 255.255.255.0
ip pim sparse-dense-mode
!
interface FastEthernet0/1
ip address 172.16.28.8 255.255.255.0
ip pim sparse-dense-mode
!
interface Serial0/1
ip address 172.16.78.8 255.255.255.0
ip pim sparse-dense-mode
!
router ospf 100
router-id 8.8.8.8
log-adjacency-changes
passive-interface Loopback0
passive-interface Loopback1
network 8.0.0.1 0.0.0.0 area 678
network 8.8.8.8 0.0.0.0 area 678
network 172.16.28.8 0.0.0.0 area 0
network 172.16.58.8 0.0.0.0 area 58
network 172.16.78.8 0.0.0.0 area 678
!
end
------------------------------------------------
其中红色字体代表RP专用配置。首先loopback口必须配置为pim sparse-dense-mode，否则下述配置报错；send-rp-announce用于向RP Agent通告RP所在，并在本地mrouting table生成(2.2.2.2,224.0.1.39)、(*,224.0.1.39)条目；send-rp-discovery用于RP Agent，用于向PIM Sparse-mode路由器通告RP所在，并生成(2.2.2.2,224.0.1.40)和(*,224.0.1.40)条目。最后由于本实验使用R1、R2作为RP，因此需要配置优先级（优先级数值越小越高）。
绿色字体代表PIM稀疏密集模式配置，包括启动ip multicast-routing、端口下启动PIM-SPARSE-DENSE模式。
紫色字体用于测试，使R8的LoopBack 1端口模拟加入组播组224.8.8.8。
Log如下：
R8#conf t
R8(config)#in l1
R8(config-if)#ip igmp join 224.8.8.8
*Mar 1 04:17:12.238: PIM(0): Check RP 2.2.2.2 into the (*, 224.8.8.8) entry
*Mar 1 04:17:12.242: PIM(0): Building Triggered (*,G) Join / (S,G,RP-bit) Prune message for 224.8.8.8
*Mar 1 04:17:12.242: PIM(0): Insert (*,224.8.8.8) join in nbr 172.16.28.2's queue
*Mar 1 04:17:12.246: PIM(0): Building Join/Prune packet for nbr 172.16.28.2
*Mar 1 04:17:12.250: PIM(0): Adding v2 (2.2.2.2/32, 224.8.8.8), WC-bit, RPT-bit, S-bit Join
*Mar 1 04:17:12.250: PIM(0): Send v2 join/prune to 172.16.28.2 (FastEthernet0/1)
*Mar 1 04:17:22.382: PIM(0): Insert (172.16.36.3,224.8.8.8) join in nbr 172.16.28.2's queue
*Mar 1 04:17:22.382: PIM(0): Building Join/Prune packet for nbr 172.16.28.2
*Mar 1 04:17:22.386: PIM(0): Adding v2 (172.16.36.3/32, 224.8.8.8), S-bit Join
*Mar 1 04:17:22.386: PIM(0): Send v2 join/prune to 172.16.28.2 (FastEthernet0/1)
*Mar 1 04:17:22.550: PIM(0): Insert (172.16.13.3,224.8.8.8) join in nbr 172.16.28.2's queue
*Mar 1 04:17:22.554: PIM(0): Insert (172.16.34.3,224.8.8.8) join in nbr 172.16.28.2's queue
*Mar 1 04:17:22.558: PIM(0): Building Join/Prune packet for nbr 172.16.28.2
*Mar 1 04:17:22.558: PIM(0): Adding v2 (172.16.13.3/32, 224.8.8.8), S-bit Join
*Mar 1 04:17:22.562: PIM(0): Adding v2 (172.16.34.3/32, 224.8.8.8), S-bit Join
*Mar 1 04:17:22.562: PIM(0): Send v2 join/prune to 172.16.28.2 (FastEthernet0/1)
R8#show ip mroute 224.8.8.8
(*, 224.8.8.8), 00:00:52/stopped, RP 2.2.2.2, flags: SJCL
Incoming interface: FastEthernet0/1, RPF nbr 172.16.28.2
Outgoing interface list:
Loopback1, Forward/Sparse-Dense, 00:00:52/00:02:07
(172.16.13.3, 224.8.8.8), 00:00:42/00:02:26, flags: LJT
Incoming interface: FastEthernet0/1, RPF nbr 172.16.28.2
Outgoing interface list:
Loopback1, Forward/Sparse-Dense, 00:00:42/00:02:17
(172.16.34.3, 224.8.8.8), 00:00:42/00:02:26, flags: LJT
Incoming interface: FastEthernet0/1, RPF nbr 172.16.28.2
Outgoing interface list:
Loopback1, Forward/Sparse-Dense, 00:00:42/00:02:17
(172.16.36.3, 224.8.8.8), 00:00:42/00:02:26, flags: LJT
Incoming interface: FastEthernet0/1, RPF nbr 172.16.28.2
Outgoing interface list:
Loopback1, Forward/Sparse-Dense, 00:00:42/00:02:17
主要观察R8 mroute条目里的Flags：第1个条目，S代表这是稀疏模式使用的条目，这是因为224.8.8.8组播组是使用RPT进行转发的；C代表其中一个接口连接组播目的地（Loopback1），L代表路由器本身就是组播成员（端口下配的igmp join-group）。关键是J。在RPT条目内，J代表该RPT已达到了SPT-Thresthold的值，将通过新生成的SPT进行转发。因此RPT条目状态已改为stopped；
而2-4个条目为SPT条目，J代表该条目为RPT超过SPT-Thresthold后生成的。若在一段时间（蓝色字体表示）内流量没超过阀值则失效。由于J标识是由RPT生成的，因此可以判断该路由器PIM为稀疏模式。
-----------------------------------------------------------
R2#debug ip pim
PIM debugging is on
R2#
*Mar 1 04:49:38.782: PIM(0): Received v2 Join/Prune on FastEthernet1/8 from 172.16.28.8, to us
*Mar 1 04:49:38.786: PIM(0): Join-list: (*, 224.8.8.8), RPT-bit set, WC-bit set, S-bit set
*Mar 1 04:49:38.786: PIM(0): Check RP 2.2.2.2 into the (*, 224.8.8.8) entry
*Mar 1 04:49:38.790: PIM(0): Add FastEthernet1/8/172.16.28.8 to (*, 224.8.8.8), Forward state, by PIM *G Join
*Mar 1 04:49:48.590: PIM(0): Received v2 Register on FastEthernet0/0 from 172.16.16.6 for 172.16.36.3, group 224.8.8.8
*Mar 1 04:49:48.598: PIM(0): Insert (172.16.36.3,224.8.8.8) join in nbr 172.16.12.1's queue
*Mar 1 04:49:48.598: PIM(0): Forward decapsulated data packet for 224.8.8.8 on FastEthernet1/8
*Mar 1 04:49:48.602: PIM(0): Building Join/Prune packet for nbr 172.16.12.1
*Mar 1 04:49:48.602: PIM(0): Adding v2 (172.16.36.3/32, 224.8.8.8), S-bit Join
*Mar 1 04:49:48.602: PIM(0): Send v2 join/prune to 172.16.12.1 (FastEthernet0/0)
*Mar 1 04:49:48.610: PIM(0): Received v2 Register on FastEthernet0/0 from 172.16.13.3 for 172.16.13.3, group 224.8.8.8
*Mar 1 04:49:48.614: PIM(0): Insert (172.16.13.3,224.8.8.8) join in nbr 172.16.12.1's queue
*Mar 1 04:49:48.618: PIM(0): Forward decapsulated data packet for 224.8.8.8 on FastEthernet1/8
*Mar 1 04:49:48.618: PIM(0): Received v2 Register on FastEthernet1/4 from 172.16.24.4 for 172.16.34.3, group 224.8.8.8
*Mar 1 04:49:48.622: PIM(0): Insert (172.16.34.3,224.8.8.8) join in nbr 172.16.24.4's queue
*Mar 1 04:49:48.622: PIM(0): Forward decapsulated data packet for 224.8.8.8 on FastEthernet1/8
*Mar 1 04:49:48.626: PIM(0): Building Join/Prune packet for nbr 172.16.24.4
*Mar 1 04:49:48.626: PIM(0): Adding v2 (172.16.34.3/32, 224.8.8.8), S-bit Join
*Mar 1 04:49:48.626: PIM(0): Send v2 join/prune to 172.16.24.4 (FastEthernet1/4)
*Mar 1 04:49:48.630: PIM(0): Building Join/Prune packet for nbr 172.16.12.1
*Mar 1 04:49:48.630: PIM(0): Adding v2 (172.16.13.3/32, 224.8.8.8), S-bit Join
*Mar 1 04:49:48.634: PIM(0): Send v2 join/prune to 172.16.12.1 (FastEthernet0/0)
*Mar 1 04:49:48.650: PIM(0): Received v2 Join/Prune on FastEthernet1/8 from 172.16.28.8, to us
*Mar 1 04:49:48.654: PIM(0): Join-list: (172.16.36.3/32, 224.8.8.8), S-bit set
*Mar 1 04:49:48.654: PIM(0): Update FastEthernet1/8/172.16.28.8 to (172.16.36.3, 224.8.8.8), Forward state, by PIM SG Join
*Mar 1 04:49:48.670: PIM(0): Received v2 Join/Prune on FastEthernet1/8 from 172.16.28.8, to us
*Mar 1 04:49:48.670: PIM(0): Join-list: (172.16.13.3/32, 224.8.8.8), S-bit set
*Mar 1 04:49:48.670: PIM(0): Update FastEthernet1/8/172.16.28.8 to (172.16.13.3, 224.8.8.8), Forward state, by PIM SG Join
*Mar 1 04:49:48.674: PIM(0): Join-list: (172.16.34.3/32, 224.8.8.8), S-bit set
*Mar 1 04:49:48.674: PIM(0): Update FastEthernet1/8/172.16.28.8 to (172.16.34.3, 224.8.8.8), Forward state, by PIM
R2#show ip mroute 224.8.8.8
(*, 224.8.8.8), 00:01:03/00:03:25, RP 2.2.2.2, flags: S
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet1/8, Forward/Sparse-Dense, 00:01:03/00:03:25
(172.16.13.3, 224.8.8.8), 00:00:53/00:02:06, flags:
Incoming interface: FastEthernet0/0, RPF nbr 172.16.12.1
Outgoing interface list:
FastEthernet1/8, Forward/Sparse-Dense, 00:00:53/00:03:25
(172.16.34.3, 224.8.8.8), 00:00:53/00:02:06, flags:
Incoming interface: FastEthernet1/4, RPF nbr 172.16.24.4
Outgoing interface list:
FastEthernet1/8, Forward/Sparse-Dense, 00:00:53/00:03:25
(172.16.36.3, 224.8.8.8), 00:00:53/00:02:06, flags:
Incoming interface: FastEthernet0/0, RPF nbr 172.16.12.1
Outgoing interface list:
FastEthernet1/8, Forward/Sparse-Dense, 00:00:53/00:03:25
--------------------------------------------------------------
R3#debug ip pim
PIM debugging is on
R3#ping 224.8.8.8
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 224.8.8.8, timeout is 2 seconds:
Reply to request 0 from 172.16.28.8, 624 ms
Reply to request 0 from 172.16.28.8, 764 ms
Reply to request 0 from 172.16.28.8, 720 ms
*Mar 1 04:45:01.846: PIM(0): Check RP 2.2.2.2 into the (*, 224.8.8.8) entry
*Mar 1 04:45:01.850: PIM(0): Send v2 Register to 2.2.2.2 for 172.16.13.3, group 224.8.8.8
*Mar 1 04:46:00.306: PIM(0): Building Periodic (*,G) Join / (S,G,RP-bit) Prune message for 224.8.8.8
*Mar 1 04:46:08.886: PIM(0): Send v2 Null Register to 2.2.2.2
*Mar 1 04:46:09.070: PIM(0): Received v2 Register-Stop on FastEthernet0/0 from 2.2.2.2 for source 0.0.0.0, group 0.0.0.0
*Mar 1 04:46:59.186: PIM(0): Building Periodic (*,G) Join / (S,G,RP-bit) Prune message for 224.8.8.8
*Mar 1 04:47:18.886: PIM(0): Send v2 Null Register to 2.2.2.2
*Mar 1 04:47:19.162: PIM(0): Received v2 Register-Stop on FastEthernet0/0 from 2.2.2.2 for source 0.0.0.0, group 0.0.0.0
R3#show ip mroute 224.8.8.8
(*, 224.8.8.8), 00:01:08/stopped, RP 2.2.2.2, flags: SPF
Incoming interface: FastEthernet0/0, RPF nbr 172.16.13.1
Outgoing interface list: Null
(172.16.13.3, 224.8.8.8), 00:01:08/00:01:58, flags: PFT
Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0, Registering
Outgoing interface list: Null
从R3的RPT条目可以看出，该条目已经Stopped，因为RP（R2）不再相应其Register消息。
本文转自 gole_huang 51CTO博客，原文链接:http://blog.51cto.com/golehuang/537792
