Subject: Re:DR and IGMP Querier relation, and Assert

From: Xin Wang <[email-masked]>
Date: Fri, 21 May 1999 01:32:26 -0400 (EDT)

I have some requestion regarding the DR router election, Assert
triggering, as well as DR and IGMP Querier relation. 

1. DR and IGMP Querier 

DR may or may not be the same router as the IGMP Querier. 
If more than one routers are connected to a LAN , 
the one with the highest IP address will be elected as
DR, and the one with the lowest IP address will be elected as IGMP
Querier. So if more than one routers on a LAN, DR and Querier 
will be different.

In this case, are there actually two Queriers on a LAN soliciting
group membership or DR only listens to the IGMP report that is solicited
by the IGMP Querier?

2. DR election and Assert

 a) 	In PIM-SM, when a router is seleted as DR, 
	it will set up multicast entries and sends 
	Join/Prune etc. When two routers A , B and C are connected
	upstream to a LAN, Assert may be needed if more than one routers have 
	multicast entries and have the LAN interface in their oif lists. 
	Assume A is DR router. The problem is, how can a second router, 
	say B, have multicast entry in the normal case? 
	One case I can think of is becase the LAN has downstream routers
	and consider B as RPF router thus send Join. 

 b) 	When Assert process settles, will any new Assert be triggered
	again afterwards? (assume no topology change and B is 
	the Assert winner, thus no Assert timer is needed)

        Two possible cases I can consider:
	------------------

 	If topology changes, router C is elected as new DR, it will
	trigger Assert to choose winner between C and B again. Thus each 
	new DR election will trigger a Assert. 

        Once the multicast entry in DR router A times out, it will create 
	new entry, and will send join again. Data packets fowarding will 
	trigger new Assert porcess. It seems the Assert is triggered
   	periodically in this case. 

 	But from another angle, if Router B (forwarder) is
	down, will DR router A knows to send join right away, or it needs
	to wait for its entry times out?
	(with pruned oif, it won't send join. If LAN is the only
	interface in its oif, the entry may time out)

        Any other cases Assert will be triggered again? 

Thanks,

Xin