Multicast Query

I have a query on multicasting that I want some clarification on. Say I have a server that is multicasting 5 video streams (each stream utilising 10Mbit/s), but nobody has joined any streams just yet, then does the feed just get sent to the switch and get dropped (i.e. wasting 50Mbits of bandwidth on the link)? Or do the applications tend not to send the feed at all until someone has actually joined the group?

Comments

  • sg4rb0,

    Imagine this like a radio.

    Even without recipient, the source will still be streaming its content.

    If this is PIM-DM, It will eat up the bandwidth pretty much 'anywhere' where multicast network is available.

    If this is PIM-SM, it will only eat up the bandwidth up to the DR.

    Thx,
    DSU


    On Fri, Jun 26, 2015 at 6:33 PM, sg4rb0 <[email protected]> wrote:

    I have a query on multicasting that I want some clarification on. Say I have a server that is multicasting 5 video streams (each stream utilising 10Mbit/s), but nobody has joined any streams just yet, then does the feed just get sent to the switch and get dropped (i.e. wasting 50Mbits of bandwidth on the link)? Or do the applications tend not to send the feed at all until someone has actually joined the group?




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  • David is correct,

     

    My knowledge of this is in PIM-SM the DR on the LAN on the segment recieves the mcast packet and creats an S,G in the mcast routing table.  It then encapsulates the packet as a PIM register message as unicast and sends it to the RP.  The RP unencaps the register packet and does one of two things.

    1. If there are recievers, *,G entries in the routing table, the RP will send PIM join messages to the source building the SPT and send the traffic down the shared tree.  

     

    2.  If there are no recievers the RP will discard the messages

     

    In either situation just like david says the the source continues to send traffic on the segment.  It is up to the DR and RP on whether or not traffic is sent up or down the tree.  IGMP snooping can help with traffic on the segment.

  • If there are no receivers, the RP only gets the traffic for the first few packets. The RP sends "Register Stop" back to the DR to ack the (S,G) being learned. So yes traffic always goes to the DR but only to the RP initially. 

    If PIM is running in Bidir mode then the RP always gets the data traffic, as it will be routed up the (*,G) to the Bidir RP. 

  • Brian, thanks for the adjustment and addition.

    Thx,
    dsu

    On Sun, Jun 28, 2015 at 12:36 AM, Brian McGahan <[email protected]> wrote:

    If there are no receivers, the RP only gets the traffic for the first few packets. The RP sends "Register Stop" back to the DR to ack the (S,G) being learned. So yes traffic always goes to the DR but only to the RP initially. 

    If PIM is running in Bidir mode then the RP always gets the data traffic, as it will be routed up the (*,G) to the Bidir RP. 




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