Why is IGMP enabled with PIM?

[ndemers]

Been doing labs and I came across a reference that said IGMP is turned on by default with PIM enablement.  I verified this but Im still not sure why.

What services does IGMP provide for PIM?

[Jon Marshall]

IGMP is used by routers to send IGMP queries and listen for IGMP responses from end hosts.

Without it there would be no way for an end device to signal it wants to receive a particular multicast stream.

When you enable PIM as you say it automatically enables IGMP because routers need to know whether a multicast stream is needed on any of it's interfaces.

Without PIM being enabled the router would not make queries for the end devices to respond to and in addition it would know what to do with any IGMP join requests it received.

Jon

[ndemers]

I should have clarified.  I understand the client router relationship with igmp.  I also understand what pim is and does.  However I dont understand why igmp is enabled on a P2P interface between two routers.

mcast svr- R1 - R2 - R3  - client

When enabling pim on interface between R1-R2-R3 igmp is enabled on those interface as well.....

why?

[Jon Marshall]

I suspect, although I don't know for sure,  this is a code issue and therefore you are probably better placed within Cisco to find the answer than me.

Simply put the code is easier if you simply assume IGMP is to be run with PIM without having to do the additional checks of what type of link it is being run on.

It does no harm to run IGMP even though it isn't being used on that specific connection.

I'm not aware of anyway to disable IGMP for PIM in the way the DR/BDR election with OSPF is bypassed if you specify the link is a point to point link.

Jon