Solved: Multicast question

John Blakley · ‎06-26-2009

All,

I've been reading on multicasting out of Jeff Doyle's book Routing TCP/IP v2. He says that "queriers are responsible for forwarding packets" and to make sure that you have a certain router the querier, you would use the lowest ip address on that segment.

Later in the book, he says that there's a forwarder election, and this forwarder is chosen by the highest ip address on that segment.

Are these two different things? Is one for PIM-DM and the other for PIM-SM?

Thanks!

John

HTH, John *** Please rate all useful posts ***

Giuseppe Larosa · ‎06-26-2009

Hello John,

they are two different concepts and roles:

the IGMP querier has the duty to send the periodic IGMP queries on the lan segment this allows all multicast routers to keep up to date the Output Interface List for all multicast groups (because IGMP reports are sent to 224.0.0.2)

the PIM forwarder is the router in charge to forward frames for a (S,G) group on a lan segment.

Competition for this role happens with the exchange of PIM assert messages:

each router advertises the AD and metric of its route to the source S.

In case of a tie the highest ip address wins.

The assert process is used by both PIM SM and PIM DM.

Actually there is a third role that is the PIM DR on segment that uses PIM priority and in case of a tie the higher ip address wins.

PIM DR has the duty to perform initial join to shared tree RPT until the source is unknown (true for IGMP version 1,2) or to send register messages to RP if the source is connected on the segment.

Hope to help

Giuseppe

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Giuseppe Larosa · ‎06-26-2009