IGMP/Multicast Help

Would someone care to explain how IGMP relates to Multicast and how this is involved in a network?
I have an idea but we are trying to implement this at the request of a software vendor to get their application to work properly in our environment.
The equip. involved are 2 6506's with 12.1(26)
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Don JohnstonInstructorCommented:
IGMP is the multicast messaging protocol between hosts and their nearest router.

By default, a router will not forward a multicast stream to a host network unless a host has requested it. The host sends an IGMP join message that is received by the router. The router will then forward the multicast stream onto that network.  Periodically, the router will send IGMP queries onto the network. If there are any hosts that are interested in the stream, they will respond. If there are none, the router will stop the stream.
IGMP is the layer two protocol used for multicast control. It serves several purposes; 1) it allows hosts to signal to routers that they do or do not want to receive a stream on that LAN subnet, or else it remains pruned and 2) it allows the router to query the hosts to see if anyone is still interested in receiving a stream.

IGMP is enabled on an interface by enabling PIM. You can validate that it is enabled by executing a sh ip igmp int.

IGMP snooping is a flood reduction feature that should be enabled on a heavy layer 2 Multicast environment. IGMP snooping allows the switch to listen to the IGMP requests and thus only send Multicast traffic to the ports requesting it, this is very important because by default Multicast is treated as a broadcast on a switch and will be flooded out every port. Broadcasting a 6Mb video stream out every port on a switch could definitely add some unneeded load especially if the end hosts are not even interested in receiving that multicast traffic.

Hope this helps,

akalbfellAuthor Commented:
Thank you both for your explanations which make perfect sense. Can you provide any help on implementing this?
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Don JohnstonInstructorCommented:
All you have to do is enable multicast routing.

ip multicast-routing
int f0/0
 ip pim sparse-dense-mode
int s0/0
 ip pim sparse-dense-mode

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akalbfellAuthor Commented:
on each interface? is there anyway to assign it to say every interface within a particular vlan?
Don JohnstonInstructorCommented:
Yes. Each layer 3 interface. Applying it to a VLAN interface will enable multicast for the entire VLAN.
akalbfellAuthor Commented:
after these changes, how can i check if all is working ok?
sh mac-address-table multicast ?
akalbfellAuthor Commented:
when trying to apply ip pim sparse-dense-mode to an interface i get an unrecognized command.
ip pim isnt even recognized and ip ? produces the following

Interface IP configuration subcommands:
  rsvp  RSVP interface commands
  rtp   RTP parameters
  vrf   VPN Routing/Forwarding parameters on the interface
Don JohnstonInstructorCommented:
Are you applying the command to a layer 3 interface?

Commands to verify multicast:
sho ip mroute
show pim interface

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