How is the IGMP querier designated?

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Nov 20th, 2008
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See, I just read the chapter on multicasting in my old BCMSN textbook, and it tells me that under IGMPv1, one router is responsible for tracking all the membership reports and querying about their multicast ambitions every 60 seconds.


But it doesn't say anything about how to configure a router to be the querier, or whether there's some automatic mechanism for that.


How does a router accede to IGMPv1 Querierhood?


Correct Answer by Edison Ortiz about 8 years 4 months ago

No, read carefully :)


If there are multiple routers on a LAN, a designated router (DR) must be elected to avoid duplicating multicast traffic for connected hosts. PIM routers follow an election process to select a DR. The PIM router with the highest IP address becomes the DR.


The DR is responsible for the following tasks:


•Sending PIM register and PIM Join and Prune messages toward the rendezvous point (RP) to inform it about host group membership.


•Sending IGMP host-query messages.


•Sending host-query messages by default every 60 seconds in order to keep the IGMP overhead on hosts and networks very low.


HTH,


__


Edison.


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Edison Ortiz Fri, 11/21/2008 - 10:45
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But it doesn't say anything about how to configure a router to be the querier, or whether there's some automatic mechanism for that.


IGMP Querier is activated in the router as soon as you enable PIM on the interface.


For more information on IGMP, please refer to the documentation:

http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_customize_igmp_ps6350_TSD_Products_Configuration_Guide_Chapter.html


HTH,


__


Edison.

CriscoSystems Fri, 11/21/2008 - 10:50
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Hmmm, book says "Every 60 seconds, one router on each network segment queries all hosts to see if they are interested in receiving multicast traffic. This router is known as the IGMPv1 Querier..."


So it ain't actually just ONE router, but rather any one with PIM enabled?


Correct Answer
Edison Ortiz Fri, 11/21/2008 - 10:54
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No, read carefully :)


If there are multiple routers on a LAN, a designated router (DR) must be elected to avoid duplicating multicast traffic for connected hosts. PIM routers follow an election process to select a DR. The PIM router with the highest IP address becomes the DR.


The DR is responsible for the following tasks:


•Sending PIM register and PIM Join and Prune messages toward the rendezvous point (RP) to inform it about host group membership.


•Sending IGMP host-query messages.


•Sending host-query messages by default every 60 seconds in order to keep the IGMP overhead on hosts and networks very low.


HTH,


__


Edison.


CriscoSystems Fri, 11/21/2008 - 11:02
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I did read carefully. The old BCMSN textbook I'm following doesn't make any mention of a DR or election process under PIM.


I guess good thing that book's out of print now.



Edison Ortiz Fri, 11/21/2008 - 11:11
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Ah, I thought you've read the link that I sent.


__


Edison.


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CriscoSystems Fri, 11/21/2008 - 11:13
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Nope, that would have been the sensible thing to do.


If you knew me better your expectations would have been different.


Joseph W. Doherty Fri, 11/21/2008 - 12:11
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BTW:

How the IGMP Querier becomes such, can differ on brand "X" routers.


Also, some switches can be an IGMP Querier. This for situations where you don't have a router, want to do multicast, and your switch supports some type of multicast flooding control (e.g. IGMP snooping).

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