I'm struggling to get my head around the idea of the DR/Queries/DF when it comes to multicast. My understanding is as follows but based on what I'm reading in the text book (TCP/IP Vol1) I'm missing something...
Querier: In IGMPv2 a Querier router is selected (if there are multiple routers on a LAN). This router (the one with the lowest IP address) is responsible for sending and listening for IGMP queries. All other routers "listen only" and will take over in the event that the querier is not heard from in the Querier timeout interval (also called the Other Querier Present Interval).
DR: Unlike IGMPv2, IGMPv1 must rely on the multicast routing protocol. So PIM-SM and PIM-DM have the idea of a Designated Router or DR. This is (and here is the important part) only relevant if IGMPv1 is in use (according to text). The elected DR is the router with the highest IP address and performs the same function as the IGMPv2 Querier.
At this point, only IGMP has been looked at... not the actual forwarding of the multicast traffic. For the forwarding of the multicast traffic (on a multi-access network), a DF is used (or designated forwarder).
DF: When one router hears multicast traffic on an interface for which that multicast group is outgoing on the interface, it will send an Assert message (no routing protocol like DVMRP to swap routes). In short, the Assert message contains information about the unicast path to the source of the multicast tree... the AD, the routing metric and the originating router IP. Lowest AD, followed by lowest metric, followed by highest IP is used to determine which router is the forwarder. The other router removes the outgoing interface from its mutlicast forwarding table. This prevents more than one router sending multicast traffic onto the multi-access network.
I think I've got that more or less accurate... but where my confusion comes in, is when I read an example in the text where IGMPv1 is not used.
Two routers are connected to a LAN with a Group Member. R1 is the IGMPv2 Querier. R2 is the DR (which shouldn't matter). R2 is the DF.
It is looking at an example of PIM-SM and RPTs (the incoming and outgoing interfaces of which I am still trying to get my head around... but thats another forum post).
The book states the following: "When a host first requests a join to a group, the DR joins the shared RP tree (RPT)". From the output everything looks as if R2 does indeed, send the join... but IGMPv1 isn't used and if anything, surely R1 as the Querier, should join the RPT....
I suppose my ultimate question is this: Router is it that will actually join the multicast tree (send join/leave or GRAFT), the IGMP Querier, the IGMP DR or the DF for the multi-access network?
It seems anything but and I'm sure I am one more study session away from having it all click and fall into place, but I could use some help to get there. Thanks in advance :)
-Steve
