igmp-snooping and many-to-many multicast

[sushansson]

Dear All,

If you have a many-to-many multicast setup with VMs.

Then several VM hosts are connected to the same physical port on the switch.

The switch is enabled for igmp-snooping.

VM-Host A--source&receiver,239.255.111.50-|

VM-Host B--source&receiver,239.255.111.50-|--Phys.SRV--link--Switch/IGMP-snooping----Router igmp querier

Can the source join its own multicast group?

This works at home on netgear switch and vlc (start two vlc apps, one for source and one for receiver on the same PC).

Develop engineers say that this does not work due to flooding technique (sometimes called split horizon) because the switch should flood to all ports except to the incoming port.

For me this is though correct for other traffic then multicast, because igmp snooping on the switch creates an igmp forwarding table with either multicast mac address or IP multicast address and therefore source should be able to join its own mgroup.

Thank you for your support in this matter,

Susanne Hansson

[Sergiu.Daniluk]

Hi @sushansson 

I think it depends on the software/hardware implementation - what platform do you have?

Also, since you have 2 VMs there behind one phy interface, I would expect to have a vswitch which will interconnect them, right? Thus shouldn't the mcast traffic be locally forwarded between them?

Stay safe,

Sergiu

[sushansson]

Hi Sergui,

Thank you for your reply,

The switch is a Nexus 3548.

You are right, the vswitch should forward the multicast traffic internally.

Whar happens if two hosts recide on the same switch port without vswitch and one of the host is the sourch whereas the other is the receiver. Will the flooding technique (split horizon) that send to all ports except incoming port apply? If yes, then the receiver host will not receive the multicast traffic because source host belongs to the same port? Is this correct assumption?

found following text 

————

Both IGMPv1 and IGMPv2 support membership report suppression, which means that if two hosts on the same subnet want to receive multicast data for the same group, then the host that receives a member report from the other host suppresses sending its report. Membership report suppression occurs for hosts that share a port.

——-

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus3548/sw/92x/multicast/b-cisco-nexus-3548_switch-nx-os-multicast-routing-configuration-guide-92x/b-cisco-nexus-3548_switch-nx-os-multicast-routing-configuration-guide-92x_chapter_0101.html

Thank you for your reply,

Susanne