The split-subnet issue arises when you divide a subnet into two unconnected sections in a network. This division results in the production of erroneous routing information about how to reach the hosts on that subnet. The split-subnet problem applies to all types of traffic, but it has a larger impact on a PIM-SM network.
To avoid the split-subnet problem in PIM networks, ensure that the RP router is not in a subnet that can become a split subnet. Also, avoid having receivers on this subnet. Because the RP is an entity that must be reached by all PIM-enabled switches with receivers in a network, placing the RP on a split-subnet can impact the whole multicast traffic flow. Traffic can be affected even for receivers and senders that are not part of the split-subnet.