Basic question on multicast

Answered Question
Oct 23rd, 2009
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Imagine I have this layout:


Source-server--->Router1----[Switch]-Receiver-client1 and Receiver-client2


In a sparse-mode configuration, IGMPv2, imagine Receiver-client1 wants to join a multicast group. Receiver-client2 is not interested. File to be transmitted is a video file, 100MB on Source-server.


Question:

Besides controlling the join/leave group notifications, prunes, etc which I understand, is the router going to buffer any part of this 100MB video at all using its resources? Or the router capability is strictly controlling the group membership behavior and sending traffic to the workstations that need it and do not sending traffic to the ones that do not need it?




Correct Answer by Jon Marshall about 7 years 6 months ago

Marlon


"Or the router capability is strictly controlling the group membership behavior and sending traffic to the workstations that need it and do not sending traffic to the ones that do not need it?"


Actually in your scenario it is the switch which filters the traffic in terms of which client receives the traffic and which doesn't, assuming you have IGMP snooping on the switch. If you don't then the switch simply sends the traffic out of all ports other than the one it was received on.


As for the router buffering, not sure what you mean exactly. The router will behave no differently with multicast traffic than it would with unicast as far as buffering etc. Is there something specific you were thinking of ?


Jon

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Correct Answer
Jon Marshall Fri, 10/23/2009 - 15:06
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Marlon


"Or the router capability is strictly controlling the group membership behavior and sending traffic to the workstations that need it and do not sending traffic to the ones that do not need it?"


Actually in your scenario it is the switch which filters the traffic in terms of which client receives the traffic and which doesn't, assuming you have IGMP snooping on the switch. If you don't then the switch simply sends the traffic out of all ports other than the one it was received on.


As for the router buffering, not sure what you mean exactly. The router will behave no differently with multicast traffic than it would with unicast as far as buffering etc. Is there something specific you were thinking of ?


Jon

Joseph W. Doherty Fri, 10/23/2009 - 18:41
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As Jon notes, the router doesn't send to individual multicast receivers, it sends to the segment. (Also as Jon notes, the switch, though, might selectively forward multicast to indvidual ports.)


As to buffering, the router might queue (and perhap drop) multicast packets as it egresses an interface. This being the case, you sometimes need to insure there's sufficient buffering capacity for something like video and/or QoS to guarantee timely forwarding.

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