Multicast Enquiry [please advise]

Unanswered Question

Understand that multicast can conserve bandwidth as a single stream of video can send to the multicast router and replicate from there (Layer 3 router).


My question is, base on my attached diagram (Fig 2), if there is an additional workstation on edge switch 1, will the multicast stream increases (from initial 1, to 2) as shown in the diagram with red arrow.


OR, its still a single stream from the distribution switch to edge switch, whereas the replication of stream is done on the edge switch to the receiver(workstation)?


The reason of me asking is that, my client is concern about the load(cpu, mem, bandwidth) on the equipments and would like us to identify the potential equipments that will have heavy load.


Please advise, Thanks



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Giuseppe Larosa Tue, 09/23/2008 - 22:51
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Hello Charles,

a single copy of each frame that are part of the video stream is sent out for each IP subnet/Vlan by the L3 device.


Cisco LAN switches have built in multicast forwarding capabilities so load shouldn't be an issue.

Multicast forwarding can be optimized on the edge switch by enabling IGMP snooping: in this case the multicast stream is sent out the access ports only if a user has sent an IGMP report for the multicast group in response to a General Query of Group specific query sent by the L3 device (they are sent every 60 seconds) the edge switch intercepts all these messages to build the L2 oilist for the multicast group.

Actually also IGMP leave messages are sniffed and used to prune the port from the L2 oilist for the group.

If your edge switch are C2950 or better I think you should be fine I would worry if they are older models like 2924XL , 3524XL and so on.


Hope to help

Giuseppe


Hi,


When you mention L2 Switches have forwarding capability, does it mean the "Single" multicast stream from the distribution switch to edge switch is replicated on the edge switch or a new stream is replicated on the distribution switch to the edge switch.


Will need your assistance to provide advise base on the diagram, as my client is very concern on where the multicast stream is replicated to the workstation/receiver.


By default, IGMP snooping is enabled. I am using 3560-POE switches


Thanks

Charles

Giuseppe Larosa Wed, 09/24/2008 - 02:32
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Hello Charles,

edge switches like yours 3560-POE have built in hardware multicast replication capabilities:


on the l2 uplink the C3650 POE receives one copy of each frame of the video stream from the L3 device.

the frame is sent out the ports where IGMP reports are heard this replication is not performed in software.

without IGMP snooping the L2 switch will send out the frame out of all ports that are in the broadcast domain /Vlan.


For the diagram the red arrow should reach only the first edge switch: the new receiver is added to the list of ports for the multicast group on the edge switch.


This is the advantage of multicast


Hope to help

Giuseppe


Hi Giuseppe,


To summarise, am i right to say there's only 1 arrow (1 stream of multicast packets) from the L3 device to my L2 edge switch, and with subsequent new receivers connect to my edge switch, the replication of multicast packets are perform on my EDGE Switch and not on my L3 device?


So on my diagram, i should remove away the RED arrow from distribution switch to Edge Switch as there will be NO new stream packets to the edge switch (Edge switch will use the initial black arrow stream of data)


Please correct me if i'm wrong.


Thanks

Charles


Giuseppe Larosa Wed, 09/24/2008 - 06:50
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Hello Charles,

yes you are correct now, change the picture accordingly


Hope to help

Giuseppe


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