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I wanted to understand how a querier election takes place.

  • Say we have to L3 devices with ip address 10.1.1.2 and 10.1.1.1.
  • PIM is not enabled anywhere.
  • I checked around and found that the flow would be as follows.
  • Say on device with ip 10.1.1.2, igmp is enabled.
  • It immediately becomes querier and starts to send query packet.
  • Then igmp is enabled on second device 10.1.1.1
  • Since it has not heard any query yet, it starts to send the query and once the first L3 device receives it, since the query that it received has an lower ip, it lets the 2nd device be the querier.

Is this how it works ?

  • Also another query that I have is the moment igmp is enabled, will it start to send query or will it wait for some time equal to X and then send ?

Thank you in advance.

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Yes - when 10.1.1.2 receives a query from 10.1.1.1 it stops being querier itself.

the moment igmp is enabled, will it start to send query or will it wait for some time equal to X and then send ?

A multicast router always starts out as a querier and continues until it receives a query with a lower IP address.

This is detailed in RFC 2236:

With respect to each of its attached networks, a multicast router may assume one of two roles: Querier or Non-Querier. There is normally only one Querier per physical network. All multicast routers start up as a Querier on each attached network. If a multicast router hears a Query message from a router with a lower IP address, it MUST become a Non-Querier on that network. If a router has not heard a Query message from another router for [Other Querier Present Interval], it resumes the role of Querier.

Additionally, when the current querier is removed from the network and isn't heard of for the [Other Querier Present Interval], the new querier is re-elected.

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  • Thank you for the explanation Zac.
    – Neil Roy
    Jun 9 at 5:46
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A multicast router always starts out as a querier and continues until it receives a query with a lower IP address.

It's worth mentioning this is only valid for IGMPv2. There is technically no querier election mechanism in IGMPv1 (from memories both devices will keep sending queries)

PIM is not enabled anywhere.

In my experience, most vendors will not separate the role of IGMP querier and PIM enabled interface. In reality, if you want to get the IGMP querier enabled, then you must enable PIM (there are different flavours). Some vendors also allow you to explicitly configure what they call "IGMP Snooping Querier" (in order for IGMP snooping to operate on a switch, you must have at least one querier). Also, another important thing worth mentioning regarding this:

Say we have to L3 devices with ip address 10.1.1.2 and 10.1.1.1.

Layer3 devices could be very vague term ... Keep in mind that only because a L3 device supports IGMP it doesn't mean it will become a querier.

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  • Thank you Danail
    – Neil Roy
    Jun 9 at 5:44

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