multicasting questions, RP, DR & Querier

Unanswered Question
Nov 14th, 2007
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just need some clarification on the roles of the RP, DR and Querier in regards to multicasting.

here is what i know (i think)

RP is used in pim sparse-mode and acts as kind of the central distribution point for the multicast stream. Anyone who wishes to receive the multicast needs to be registered with the RP. RP can be statically assigned or auto using the send-rp-announce and send-rp-discovery commands.

One question i have is, if using auto-rp, hows is the RP chosen, in all my labs it has been the router with the highest IP, so is this the case?

The querier is the router on the network that sends messages to see if any hosts are still interested in the multicast. It is the router with the lowest IP address. Correct?

Im confused as to the role of the DR... im assuming its similar to the DR in OSPF, but then that sounds very similar to the RP..

can someone please clarify these roles for me, or maybe point me to a link with a good description?


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Kevin Dorrell Wed, 11/14/2007 - 04:32
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I am no expert on multicast, but I shall have a go at answering the question and if I get it wrong someone will correct me.

The RP is indeed a sort of central distribution point in sparse mode. But there is one further point to be considered. Think of a source that is being picked up by the RP, and a listener that has registered its interest in that stream at the RP. The listener hears the stream from the RP - this is known as "shared tree". But once the listener sees the source of the stream, it may realise that there could be a more efficient way through the network. At that point, it sends out protocol that causes that more efficient path to be activated between that specific source and that specific listener, maybe bypassing the RP altogether. In other words, it switches from a shared tree to s shortest-path tree.

Bi-dir is like sparse-mode, but always goes through the RP. It never switches over to the shortest-path tree.

Now, auto-rp is indeed the highest IP. One point about auto-rp is that it uses a multicast to elect the RP. So sparse mode would be useless for that election because there is no RP yet. That is why when you use auto-rp you should also use sparse-dense-mode. What sparse-dense mode does is that for each multicast group it acts like dense mode until an RP has been elected, then it switches to sparse mode.

The querier - you have it correct.

The DR is only relevant when you have more than one multicast router on a network segment. It decides which of them will transmit the stream onto the network. It I remember rightly, it is the one closest to the source, with lowest IP as a tie-breaker.

Would someone like to correct my assignment please? ;-)

Kevin Dorrell



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