Multicasting IGMP and PIM

IGMP = Layer 2 protocol for devices in the same subnet to JOIN to a multicast
PIM = Layer 3 protocol to join subnets to the common multicast

IGMP version 1 sends messages to 224.0.0.1 (all hosts multicast group)
IGMP version 2 sends messages to 224.0.0.2 (all routers multicast group)

All routers will ignore the JOIN message, except the Designated Router (DR).

For routers to forward multicast use:
    ip multicast-routing

For router interface to listen for IGMP (which joins election for DR)
    interface XXX
      ip pim (sparse-mode | dense-mode | sparse-dense-mode)

Sparse Mode = Uses rendezvous  point to reduce bandwidth usage.
Dense Mode = Multicast packets from a source are flooded to all areas of a PIM-DM network.

INTERFACE MULTICAST SUBSCRIPTION

interface XXX
 ip igmp join-group
  or
  ip igmp static-group  

join-group = The router will join the multicast IP, and therefore "process switches" the data.
static-group = The router "fast switches" the multicast data to the interace. Saves CPU

SHOW COMMANDS

sh ip igmp interface - Shows interface's IGMP settings, IGMP joins, and the IGMP designated router (DR)
sh ip igmp membership (or groups) - Shows multicast groups, the router is routing for (ie propagating)


sh ip pim rp - Shows rendezvous point for PIM in sparse (or sparse-dense) mode
sh ip pim neighbor - Shows neighbours also running IGMP/PIM
sh ip pim interface - Show interfaces running IGMP/PIM


sh ip rpf x.x.x.x - Shows return parth to get the to the source of the multicast

sh ip mroute - Show sources of mutlicast
sh ip mroute counter - Shows multicast traffic stats (packets received and forwarded)
sh ip mroute active - Shows throughput of all active multicasts

More Reference
Cisco Multicast Troubleshooting Guide