How to force one router to be igmp snooping querier "master"?

Unanswered Question
Jul 6th, 2007

Hi all


I have two 6513 switches connected over a L2 trunk. Each switch has an SVI for the same subnet and HSRP configured with one switch being the active and the other standby. Both switches has the command " ip igmp snooping querier" configured on the SVI.


Problem;

When I run the "show ip igmp snooping mrouter" command on the primary switch I the the answer that the mrouter resides on the trunking port connected to the standby switch. When I run the same command on the secondary switch I get the answer that the "switch" is the mrouter.


My question is this;

Is this "election" of igmp snooping querier mrouter a random process or something that can be managed? I.E. can I set the primary switch to be the snooping querier mrouter "master"?


Regards

Fredrik Hofgren


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Jon Marshall Fri, 07/06/2007 - 08:54

Hi Frederik


Which of your switches has the lowest IP address on the SVI (not the HSRP address). I believe it is the interface with the lowest ip address that is elected the querier.


I manage to run a quick test in our lab on 6500 and this was the behaviour i saw but i didn't have time to swap around IP addresses and see if the querier swapped over.


Jon

hoffa2000 Fri, 07/06/2007 - 09:03

Intresting. However this doesn't match my system where the elected querier has a higher IP than the active router. It might be some MAC address thats used when electing the querier though.


Fredrik

Jon Marshall Fri, 07/06/2007 - 09:16

Frederik


One other thought which i will test next week.


You can modify


1) igmp query-interval - how often the querier queries for group members - default 60 seconds


2) igmp query-timeout - how long a router will wait to hear a query before taking over as querier - default 2 x igmp query-interval.


If you left the defaults on secondary switch or evne increased them but reduced the query-interval on primary switch to say 15 seconds the primary switch would wait 30 seconds and then take over.



As i say, would need testing. Not sure without further reading whether the IGMPv2 packets need to agree on timers or not ad not sure of extra processing overhead.


Just a thought.


Jon


darrenj Sat, 07/07/2007 - 03:55

Hello guys. It depends what version of IGMP you are running. With IGMP v2 and v3, the router with the lowest IP address becomes the elected querier. IGMP v1 doesn't elect one!


Hope this helps.

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