Multicast for any-to-any application in LAN

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Aug 10th, 2010
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Hello,


soon we will get a compute cluster with multicast for the distribution of files between the cluster members.


In the first step the application will stay local to one subnet, in a second step it will span over different LANs and locations.

All clients are also sources and not just receive traffic from the multicast group, but also send traffic to the group.




What I am not sure about is for the first step, do I need IGMP enabled? The clients send IGMP join messages, but do I really need an IGMP querier? As the multicast traffic stays inside the network/VLAN, I think I do not need it.


For the second step, when the clients span multiple networks, do the clients need to register as sources at the multicast routers? I would imagine from the router perspective it looks like source-specific multicast.


I was looking for documentation about this, but there is not a lot. If you have a good document, please send me a link.


Thank you

/Nova

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Giuseppe Larosa Tue, 08/10/2010 - 12:12
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Hello Nova,


>> he clients send IGMP join messages, but do I really need an IGMP querier? As the multicast traffic stays inside the network/VLAN, I think I do not need it.

An IGMP querier will allow IGMP snooping to work is highly recommended as I would not disable IGMP snooping in the vlans or all multicast packets would be treated as broadcast.

So an IGMP querier if your switch supports it is the right tool to optimize forwarding and keeping traffic into a single vlan/broadcast domain.


IGMP snooping is enabled by default in modern IOS LAN switches.


>> For the second step, when the clients span multiple networks, do the clients need to register as sources at the multicast routers? I would imagine from the router perspective it looks like source-specific multicast.


For any to any the recommended PIM mode is bidirectional PIM not PIM SSM.


your scenario is a perfect fit for bidirectional PIM that uses RP(s).


In bidirectional PIM routers with a multicast source connected are allowed to send packets back to the RP


in unidirectional PIM packets are sent only in the direction that goes away from RP so the need for register messages and then for switchover to source based tree.


Hope to help

Giuseppe

novanova99 Tue, 08/10/2010 - 13:46
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Hi Giuseppe,


thank you for your reply.


An IGMP querier will allow IGMP snooping to work is highly recommended as I would not disable IGMP snooping in the vlans or all multicast packets would be treated as broadcast.

So an IGMP querier if your switch supports it is the right tool to optimize forwarding and keeping traffic into a single vlan/broadcast domain.


I agree, but as I do have all clients talking to all other clients, the IGMP snooping is not bringing much benefit in this specific case. But it is cleaner in any case.

What I am not sure about is what the "job" of the IGMP querier in this scenario is supposed to be, all the traffic is created locally and is send locally. So even if the IGMP querier is getting queries, what is it doing with them?


Group A is local to the LAN, client1 sends a message to join group A and the message arrives at the querier. What happens then?

Does the querier try to get the group upstream? Is there any way to let the querier know, that the source(s) are local as well?


For any to any the recommended PIM mode is bidirectional PIM not PIM SSM.


your scenario is a perfect fit for bidirectional PIM that uses RP(s).


Thanks for reminding that, that's indeed the way to go.


Thanks

/Nova

Giuseppe Larosa Tue, 08/10/2010 - 14:24
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Hello Nova,

>> Does the querier try to get the group upstream? Is there any way to let the querier know, that the source(s) are local as well?


IGMP snooping acts creating an OSI layer2 olist for a given group G and does this by snooping= listening to IGMP reports of interested receivers.


IGMP reports are sent as answer to IGMP queries, and from this comes the need for an IGMP querier if no PIM router is active in the vlan/broadcast domain.

Without a querier a receiver sends just one unsolicited IGMP report so it would removed from IGMP snooping oilist within 2-3 minutes.

With a querier a query is sent every 60 seconds (or 120 seconds may be in newer IGMP versions I haven't checked)


So an IGMP querier is needed when you want to take advantage of IGMP snooping forwarding optimization but you don't want to perform multicast routing (pim) towards other IP subnets


Hope to help

Giuseppe

novanova99 Tue, 08/24/2010 - 13:47
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Hi Giuseppe,


thanks again for your response. I think you misunderstood my last comment.


What I am not sure about is what happens when I have a LAN where the sources and the receivers are connected and I have a router as an IGMP querier.


What happens when the receivers send a membership report for a group where the source is on the local LAN? Will the router still try to get the group (that is local) upstream?


Like I said in my initial post, in my scenario everything is in the same subnet/VLAN.


Thanks

-Nova

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