PIM DR vs IGMPv2 Querier

[CSCO11598534]

Hello all,

It is known that the router with the highest IP address in a segment becomes the PIM DR.

It is also know that the router with the lowest IP address in a segment becomes the IGMP querier.

Well, I know that the job of the PIM DR in a source segment is to register with the RP. Fair enough.

What about the receiver portion. I have read that PIM DR is responsible for sending Join messages to the RP.

If this is the case, then the IGMP querier and PIM DR will be different routers in this segment!

So one router hears that a receiver exists in the segment and another one is sending the Join to the RP???

[Joseph W. Doherty]

"So one router hears that a receiver exists in the segment and another one is sending the Join to the RP???"

I recall IGMP snooping doesn't block the port to a router, so the DR would "hear" at least one receiver reply. If it does, then all would work fine, although perhaps not as optimally as if the querier and DR were the same device.

[CSCO11598534]

Thanks for answering.

Well, can someone verify that my understanding is correct? If this is the case then it is very inefficient right?

I would expect the same router to do both jobs.

[Joseph W. Doherty]

How do you figure it's "very inefficient"?

[CSCO11598534]

Thinking about that again, yes it is not insufficient. It is a bit odd, but it should work.

My last question though is when igmp snooping is enabled. I have read this:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SY/configuration/guide/sy_swcg/ipv4_igmp_snooping.html#14163

I get that the switch will allow the IGMPv2 Query from port 1 (IGMP Querier).

But how does it know that it should forward the IGMPv2 Join to e.g. port 10 (PIM DR) ?

[Joseph W. Doherty]

I would have to dig for the answer, but I recall that somehow the querier "knows" what port is used by a mulitcast router and insures it gets at least one IGMP reply.

Or, it might be as simple as the querier floods a reply to all ports that haven't also replied.

[CSCO11598534]

I am sorry i dont get you. I suppose by saying "querier" you mean the switch?

I suppose this is a case that if the two roles described belonged to the same router of a segment, then we wouldnt have unnecessary flooding.

Regardless, i just wanted to know that indeed my understanding (for lowest/highest IP) is correct.

[Joseph W. Doherty]

By querier, I mean the device that queries hosts that want to continue to receive a multicast stream. A function normally done by a multicast router, but some switches can perform that function too.

As to lowest/highest IP usage, I always forget what's what, unless I look it up when I have a need to know (or I'm studying for an examine).

[CSCO11598534]

Hello,

Regardless, my concern is if indeed in IGMPv2 we have two elections:

1) The lowest IP router of segment becomes the IGMPv2 querier

2) The highest IP router of segment becomes the PIM DR

Can someone verify this please?