PIM = Layer 3 protocol to join subnets to the common multicast
IGMP version 1 sends messages to 126.96.36.199 (all hosts multicast group)
IGMP version 2 sends messages to 188.8.131.52 (all routers multicast group)
All routers will ignore the JOIN message, except the Designated Router (DR).
For routers to forward multicast use:
For router interface to listen for IGMP (which joins election for DR)
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
ip igmp join-group
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
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
Cisco Multicast Troubleshooting Guide