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I'd like to verify if I understand multicasting correctly. In a pim sparse-mode configuration, the RP rtr becomes the "root" of the stream for the rest of the configured routers. Does this mean that destination hosts clients will get their stream directly from that rp router? Or will they get the stream from their closest pim router?

  [pim rtr] <---> [pim rtr] <---> ["RP rtr"] <---> [pim rtr] 
      |                                                |
[source host]                                  [destination host]
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up vote 4 down vote accepted

Joins are sent initially to the rendezvous point set up for a particular range of groups. After the multicast tree to the RP is formed generally a shortest path tree (SPT) is formed based on IGP metrics. Traffic is then pruned down from the RP-based tree to the new SPT.

In other words, the RP is initially part of forwarding the traffic but can be (and often is) cut out shortly after the initial group of listeners is identified.

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So if I have 1000 users trying to watch a stream, all 1000 will hit the RP but eventually (or soon) they will get their stream from their closest router, right? – mrtechalot May 31 '12 at 15:15
If those thousand users are somehow in a thousand subnets on a thousand different routers! <grin> Within a given subnet if the group is already active then obviously more stations can join without anything hitting the RP. By the same token if a new subnet sends a join for a group that's already registered in PIM on the same router then the RP also isn't hit. Also - this is where anycast starts to come in. A protocol like MSDP is used to synchronize state amongst a number of routers, each of which has a loopback with the same RP address, thus allowing for HA and scaling. – rnxrx May 31 '12 at 21:02
Are you talking about Auto-RP? I only did a basic pim-sparse-mode configuration on our routers and L3-switches with our core router being the RP. I don't know if Auto-RP will help much. The other consideration is if we want to stream from one of our furthest sites, wouldn't that be too much of a hit to our RP? It will be Anycast to the RP (from the client), and anycast from the RP to the rest of the routers that having clients asking to join a group, right? ( I feel like am getting this ) – mrtechalot May 31 '12 at 21:17
No - the anycast/MSDP mechanism I described is an alternative to autorp (which I'm not a fan of, personally). The way the anycast would work is that all of the potential RP's in the network would be synchronizing the groups they knew about. Remember - each anycast RP has two loopbacks - its primary plus the anycast. When a client router sends a PIM request to its configured RP then it will just go to whichever is the nearest. The distance of the router with the source doesn't matter much. When the SPT is formed then new joins will be grafted on as needed - no hit to the RP, really. – rnxrx May 31 '12 at 21:45
Oh!, anycast RP. Thanks. – mrtechalot May 31 '12 at 22:00

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