1206 CHAPTER 71: PIM C ONFIGURATIONthe router does not have a router to the multicast source, or if PIM-DM is notenabled on the router’s RPF interface to the multicast source, the router cannotcreate (S, G) entries.■ When a multicast router receives a multicast packet, it searches the existingunicast routing table for the optimal route to the RPF check object. Theoutgoing interface of this route will act as the RPF interface and the next hopwill be taken as the RPF neighbor. The RPF interface completely relies on theexisting unicast route, and is independent of PIM. The RPF interface must bePIM-enabled, and the RPF neighbor must also be a PIM neighbor. If PIM is notenabled on the router where the RPF interface or the RPF neighbor resides, theestablishment of a multicast distribution tree will surely fail, causing abnormalmulticast forwarding.■ 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 thecorresponding interface of the RPF neighbor router, the establishment of amulticast distribution tree will surely fail, causing abnormal multicastforwarding.■ The same PIM mode must run on the entire network. Otherwise, theestablishment of a multicast distribution tree will surely fail, causing abnormalmulticast forwarding.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.2 Check that PIM is enabled on the interfaces, especially on the RPF interface. Usethe display pim interface command to view the PIM information on eachinterface. If PIM is not enabled on the interface, use the pim dm or pim smcommand to enable PIM-DM or PIM-SM.3 Check that the RPF neighbor is a PIM neighbor. Use the display pim neighborcommand to view the PIM neighbor information.4 Check that PIM and IGMP are enabled on the interfaces directly connecting to themulticast source and to the receivers.5 Check that the same PIM mode is enabled on related interfaces. Use the displaypim interface verbose command to check whether the same PIM mode isenabled on the RPF interface and the corresponding interface of the RPF neighborrouter.6 Check that the same PIM mode is enabled on all the routers in the entire network.Make sure that the same PIM mode is enabled on all the routers: PIM-SM on allrouters, or PIM-DM on all routers. In the case of PIM-SM, also check that the BSRand RP configurations are correct.Multicast DataAbnormally Terminatedon an IntermediateRouterSymptomAn intermediate router can receive multicast data successfully, but the data cannotreach the last hop router. An interface on the intermediate router receives data butno corresponding (S, G) entry is created in the PIM routing table.
