The most common reasons that routers fail to forward multicast (mcast, multi-cast, PIM) packets are due to Reverse Path Forwarding (RPF) failures and Time-to-Live (TTL) failures.
When troubleshooting multicast routing, the primary concern is the source address. Multicast routing won't forward a packet unless the source of the packet passes a Reverse Path Forwarding (RPF) check. Once a packet passes this RPF check, multicast routing forwards the packet based only upon the destination address. When a multicast packet arrives on an interface, the RPF process checks to ensure that this incoming interface is the outgoing interface used by unicast routing to reach the source of the multicast packet. This RPF check process prevents loops.
The following document walks you through the process of troubleshooting various problems based on case studies. You'll see which commands we use to quickly pinpoint the problem and you'll learn how to resolve it. The case studies used in the document are generic across the protocols, except for the IP Multicast Troubleshooting Guide.