Multicast without an mrouter

Hi all,

After reading a Cisco article on IGMP snooping (http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008059a9df.shtml) I was interested to see what happened to multicast in a Layer 2 network with no mrouter configured on the subnet. The Cisco article seems to suggest the IGMP snooping mechanism "breaks down" in the absence of an mrouter and traffic gets dropped.

So to test their claim I setup a simple test network:

PC1-----3550-----3560-----PC2

PC1:
msender.exe 239.6.6.6 10000

PC2:
mreceiver.exe 239.6.6.6 10000

(For anyone wanting to play around with multicast in their CCIE studies I would highly recommend the command line tools available here: http://www-personal.umich.edu/~bdr/et/mcast-windows.html)

I found that multicast works OK at Layer 2 in the absence of an mrouter (at least on 3550s and 3560s). The switches simply flood the multicast traffic on all ports in the VLAN (not ideal but at least it works). As soon as you introduce an mrouter on the subnet (SVI with PIM enable for example), IGMP snooping starts to work and multicast is constrained to interested hosts.

Unfortunately I don’t have any 3750s or 6500s to test further. Does anyone know the default behaviour for multicast on those platforms?

I would love to know if this flooding behaviour is consistent for all Catalyst/Nexus platforms. Or do some Cisco switches "fail closed" for multicast traffic in the absence of an mrouter/IGMP querier?

Thanks

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