At the bottom of page 5 of RFC 2236, it says that when a Querier receives a Leave, it sends [Last Member Query Count] group-specific queries and "any Querier to non-Querier transition is ignored during this time; the same router keeps sending the group-specific queries." I interpret that to mean that even if another router with a lower IP sends a Query, the group-specific queries will continue HOWEVER, in all other ways, the Querier will cease being a querier and when the group-specific queries from the Leave are done, it will be a non-Querier. The alternative, that the Querier ignores Queries from routers with a lower IP while it's processing the Leave inappropriate. Does anyone here have an opinion, alternate interpretation, or resource they can point me to for clarification? Thanks.
Hmmm. In RFC 2236, under normal circumstances, there is only one querier per "physical network" (which means per IP subnet, in this context). The querier is the multicast router with the lowest IP address. So, normally, no multiple queriers.
Multicast routers determine which one to make the querier by monitoring the "all hosts" queries in the IP subnet. So they are all listening for
224.0.0.1 query multicasts.
In Section 7, it also states that multicast routers may be in one of four states, which applies to queriers and non-queriers, the way I read it. The four states involve knowledge of membership in all multicast groups available to that IP subnet. This means that each multicast router monitors the group address of all multicast groups available. Because that's what IGMP reports are sent to.
IGMP "leave" reports are sent to the all routers group, 224.0.0.2, which means that all multicast routers will see these leave reports as well.
So finally, if there is to be no transition from "querier" to "non-querier" state while the querier is sending out group-specific queries, and if all other multicast routers are fully aware of what's going on, I would conclude that no other router will assume the querier role during this interval, under normal circumstances. Because there's only supposed to be one querier in any IP subnet.
But of course, a new multicast router just coming on line would be a querier no matter what was going on. That's how a multicast router first comes on line. And this new guy is not required to shut up if it hears a query from any router with a greater IP address. So it's certainly possible that multiple queriers will coexist for these short transitional periods. And fortunately, this causes no harm.
I'd say it differently. I'd say that under normal circumstances, only one querier exists. But in certain cases, the querier waiting for the "leave group" timeout to occur will ignore any other queries, from a router with lower IP address. so that multiple queriers might exist in these periods.
I'll bet that this is implemented differently in different routers, and that the differences are inconsequential.
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