PIM-SM RP Function

In studying PIM-SM I'm having trouble figuring out exactly what the function of the RP is... Please tell me if I am correct.

First off in Dense mode, the traffic is flooded everywhere and then pruned later if no one wants that traffic. This is not the case in SM and is why we configure SM. I believe the function of the RP is to help prevent this by telling the router asking about a certain group, where to find the specific multicast address. I'm trying to dumb this down as much as possible without any lingo so I can truly understand this. It's driving me crazy... Please let me know if I am correct in my very basic explanation.

Thanks.

Comments

  •  

    acsbmx_1,

     

     

    It is easy.

    Can you drink?[B]

    Suppose you are in a beer party.

    The RP functions as a beer server.

     

    Those who would like to drink, go to the beer server.

    Those who would not, keep silent.

    This is the PIM-SM.

    It prevents the flooding of beer…oops, data and associated waste of resources.

     

    On the flip side, PIM-DM is just like an ordinary bar.

    Put yourself in lightweight's shoes.

    Do you want to repeatedly be asked “How about beer?” by a bar girl?

    It is a waste of your energy to say “No, thanks” every time.

    So is her.

     

    HTH:

    http://www.cisco.com/en/US/products/ps6604/products_white_paper09186a00800a3db6.shtml#wp38828

     

  • The RP is there so the receivers can learn about the sources.

     

    Receivers signal with IGMPv2 that they want traffic for a particular group, but they don’t know the source.  The router on the segment takes this IGMPv2 Report (the “join”) and sends it to the RP.  Now the RP knows about the receivers.  This is called the “shared tree” or (*,G) tree. 

     

    On the other end, when a senders starts to send multicast, the router attached to it tells with RP with the PIM Register message.  Now the RP knows about the sender.  This is called the “source tree”, “shortest path tree”, or (S,G) tree.

     

    The RP then builds the tree end-to-end so the receiver can start getting the traffic.  Traffic flows from the source to the RP on the (S,G) tree, and then from the RP to the receiver on the (*,G) tree.  Once this is complete the receiver now knows where the sender is, and can join a shortest path tree (S,G) tree directly to them.

     

     

    Brian McGahan, CCIE #8593 (R&S/SP/Security), CCDE 2013::13

    [email protected]

     

    Internetwork Expert, Inc.

    http://www.INE.com

     

    From: [email protected] [mailto:[email protected]] On Behalf Of acsbmx_1
    Sent: Thursday, February 21, 2013 2:52 PM
    To: Brian McGahan
    Subject: [CCIE R&S] PIM-SM RP Function

     

    In studying PIM-SM I'm having trouble figuring out exactly what the function of the RP is... Please tell me if I am correct.

    First off in Dense mode, the traffic is flooded everywhere and then pruned later if no one wants that traffic. This is not the case in SM and is why we configure SM. I believe the function of the RP is to help prevent this by telling the router asking about a certain group, where to find the specific multicast address. I'm trying to dumb this down as much as possible without any lingo so I can truly understand this. It's driving me crazy... Please let me know if I am correct in my very basic explanation.

    Thanks.




    INE - The Industry Leader in CCIE Preparation
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  • With the crystal clear explanantion given by Brian above, I'm pretty sure this won't be necessary. However, if you want a really dumbed down answer, here's one from Dictionary.com:



    ren·dez·vous
    /'r?nimaged??vu, -de?-; French r?~imaged?'vu/ Show Spelled [rahn-duh-voo, -dey-; French rahn-de-voo] Show IPA noun, plural ren·dez·vous /-?vuz; French -'vu/ Show Spelled [-vooz; French -voo] Show IPA , verb, ren·dez·voused /?vud/ Show Spelled [vood] Show IPA , ren·dez·vous·ing /-?vuimage??/ Show Spelled [-voo-ing] Show IPA .


    noun
    1.
    an agreement between two or more persons to meet at a certain time and place.


    2.
    the meeting itself.


    3.
    a place designated for a meeting or assembling, especially of troops or ships.


    4.
    a meeting of two or more spacecraft in outer space.


    5.
    a favorite or popular gathering place.










    In other words, RP is a pre-arranged place for the receivers to meet the senders. This arrangement is benefitial and even necessary in PIM Sparse mode because the receivers [EDIT] and the senders tend to live sparsely from each other! :)





  • I like to think of it this way:

    With pim-sparse (non-SSM), the multicast sources are unaware of who is requesting their traffic.  Their job is to send traffic out, not to concern themselves with where it is going, or how it gets there.  The sources send the traffic to the router, which then sends it to an RP.  The RP then adds a (S,G) entry to its mroute table, so it now knows when someone wants to join that multicast group, this is where to go.  A receiver then says hey, I want traffic from this multicast group (G).  I don't know what the unicast source is, but my router has an RP that should know where this traffic comes from.  I send an IGMP join message to the RP saying that I want to receive traffic from a certain (*,G).  The RP is where the two meet, per se, and a multicast stream is formed. 

    Yes, that's simplifying it probably too much, and I'm still learning multicast so my explanation might not be 100%.  Hope that makes sense.

  • Ok, good explanation guys. I think I've got it now. I was getting confused by how the RP could provide the end destination, but the traffic could actually take a different path to reach that destination. Let me draw out how I see this, and make sure I'm understanding it. (I'm looking at the WB1 diagram)

    So I want to send to 224.0.0.1 from R4. The RP is R5, and the one hosting 224.0.0.1 is SW3. Obviously my preferred path to reach SW 3 would be through the ethernet links. However I have to send the requested traffic to R5 to find out where 224.0.0.1 resides. So my request gets sent out and R5 says, oh... 150.1.9.9 hosts that address. So R4 says ok sweet, looks at it's IGP table and routes the traffic that way? Am I understanding this clearly?

  • Not sure I'm understanding your question.  Are you saying that R4 is the sender, or is SW3?

  • SW3 would be the sender in that scenario. Sorry.

  • As the sender, SW3 would first send it's PIM Registration message to the RP, which will contain the (S,G) information.  This information would then populate the mroute table of the RP.  The receiver, R4, would then generate and IGMP join request with the (*,G) information and send it to the RP.  The RP would then generate a shared tree or shortest-path tree.  This created a shortest path tree which is sent to the multicast group members and now the receiver would know how to reach the source.  Think of the RP as a point that connects the source to a blind receiver.  The receiver doesn't know how to get to the source, it just knows what information it wants to receive.  The RP brings them together and tells the receiver how to reach the source.  That's my understanding, which I will stress could be flawed at this point in my studies. 

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