Operation Manual – MulticastH3C S7500E Series Ethernet Switches Chapter 5 PIM Configuration5-525.8 Troubleshooting PIM Configuration5.8.1 Failure of Building a Multicast Distribution Tree CorrectlyI. SymptomNone of the routers in the network (including routers directly connected with multicastsources and receivers) has multicast forwarding entries. That is, a multicast distributiontree cannot be built correctly and clients cannot receive multicast data.II. Analysisz When PIM-DM runs on the entire network, multicast data is flooded from the firsthop router connected with the multicast source to the last hop router connectedwith the clients along the SPT. When the multicast data is flooded to a router, nomatter which router is, it creates (S, G) entries only if it has a route to the multicastsource. If the router does not have a route to the multicast source, or if PIM-DM isnot enabled on the router’s RPF interface to the multicast source, the routercannot create (S, G) entries.z When PIM-SM runs on the entire network, and when a router is to join the SPT, therouter creates (S, G) entries only if it has a route to the multicast source. If therouter does not have a router to the multicast source, or if PIM-DM is not enabledon the router’s RPF interface to the multicast source, the router cannot create (S,G) entries.z When a multicast router receives a multicast packet, it searches the existingunicast routing table for the optimal route to the RPF check object. The outgoinginterface of this route will act as the RPF interface and the next hop will be takenas the RPF neighbor. The RPF interface completely relies on the existing unicastroute, and is independent of PIM. The RPF interface must be PIM-enabled, andthe RPF neighbor must also be a PIM neighbor. If PIM is not enabled on the routerwhere the RPF interface or the RPF neighbor resides, the establishment of amulticast distribution tree will surely fail, causing abnormal multicast forwarding.z Because a hello message does not carry the PIM mode information, a routerrunning PIM is unable to know what PIM mode its PIM neighbor is running. Ifdifferent PIM modes are enabled on the RPF interface and on the correspondinginterface of the RPF neighbor router, the establishment of a multicast distributiontree will surely fail, causing abnormal multicast forwarding.z The same PIM mode must run on the entire network. Otherwise, the establishmentof a multicast distribution tree will surely fail, causing abnormal multicastforwarding.III. Solution1) Check unicast routes. Use the display ip routing-table command to checkwhether a unicast route exists from the receiver host to the multicast source.
