Edison Ortiz Mon, 12/10/2007 - 14:23

Unlike IGMPv1, in which the DR and the IGMP querier are typically the same router, in IGMPv2 the two functions are decoupled. The DR and the IGMP querier are selected based on different criteria and may be different routers on the same subnet. The DR is the router with the highest IP address on the subnet, whereas the IGMP querier is the router with the lowest IP address.

Query messages are used to elect the IGMP querier as follows:

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.

3. All routers (excluding the querier) start the query timer controlled by the ip igmp querier-timeout command, which is reset whenever a general query message is received from the IGMP querier. If the query timer expires, it is assumed that the IGMP querier has gone down, and the election process is performed again to elect a new IGMP querier.

By default, the timer is 2 times the query interval controlled by the ip igmp query-interval command.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/himc_c/chap10/mcbcigmp.htm

ellendrew Mon, 12/10/2007 - 14:38

Hi Edwin,

Thanks for the prompt reply. But what if there is 2 IGMP router on the same broadcast domain. Will the lowest IP prevail?

Thanks,

Drew

Correct Answer
Edison Ortiz Mon, 12/10/2007 - 14:40

I believe I addressed this on my previous reply, the answer is Yes.

Actions

This Discussion