BSCI - Multicasting Designated Querier question

Unanswered Question
Oct 22nd, 2007

found some conflicting statements regarding the Querier election in IGMPv2.

in the train signal multicasting section Chris Bryant says that the "router with the lowest IP address is the querier"

in the CCNP self study book on page 604?: "router with the highest IP address is the designated querier"

in the CCNP official exam cert guide on page 484: "all routers start as queries and transition to non queries if they hear another querier with a higher IP address"

also the cisco web site says this:

"For IGMP Versions 2 and 3, the designated querier is the highest IP-addressed multicast router on the subnet"

http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r2.0/multicast/command/reference/3mrigmp.html

Im just confused cuz a few prastice questions i have seen have the correct answer as the lowest IP.

can someone please clarify this for me?

thanks

I have this problem too.
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ankbhasi Tue, 10/23/2007 - 01:38

Hi Luca,

Router with the lowest ip address on that subnet will become querier.

1. When IGMPv2 routers start, they each multicast a general query message to the all-systems group address of 224.0.0.1 with their interface address in the source IP address field of the message.

2. When an IGMPv2 router receives a general query message, the router compares the source IP address in the message with its own interface address. The router with the lowest IP address on the subnet is elected the IGMP querier.

Also from RFC 2236 section 3 , 3rd para

"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"

http://www.faqs.org/rfcs/rfc2236.html

HTH

Ankur

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LucaSalvatore_2 Tue, 10/23/2007 - 14:39

so whats the deal with all these different cisco texts quoting that the querier is the highest IP address?

ankbhasi Tue, 10/23/2007 - 21:25

Hi Luca,

It's wrong when it says higher ip address will be selected as querier. Even when you check CCIE TCP/IP vol 1 by Jeff Doyl you will see lower ip address will be selected as designated querier for IGMP v2.

I have sent an email to the link which you have provided to get it rectified. Also you can send an email to the author of the book to get it rectified.

HTH

Ankur

*Pls rate all helpfull post

LordFlasheart Tue, 10/23/2007 - 23:05

Hi,

I have seen something which says to not confuse the designated querier with the designated router as their election is different. DR is highest, DQ is lowest, therefore a router can be a DR and/or a DQ, or neither.

Regards,

Chris

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