Multicast Forwarding Toward Multicast Router

[ben.wiechman]

I've been having a hard time finding an explanation for why or how a L2 switch forwards multicast traffic toward a multicast router in a specific scenario. 

Scenario

• Multicast source
• One or more L2 switches between the multicast source and FHR
• There exists a functional PIM domain of one or more additional routers beyond the FHR router that are attempting to join the multicast stream

In this scenario, what mechanism causes the L2 switch to forward the multicast traffic toward the FHR. The router doesn't send an IGMP join does it? 

Should the L2 switch forward all multicast traffic toward the multicast router? 

I've been trying to find this particular scenario, but all the examples available tend to focus on the PIM domain downstream of the RP, or focus on the mechanisms used by multicast receivers to join streams when one or more L2 switches exist between the LHR and the receiver.

I have not been able to find a discussion of the process used by L2 switches to forward traffic from a multicast source toward the DR/FHR. 

[Peter Paluch · Accepted solution]

Hi Ben,

The answer is extremely simple, and it follows from the basic operation of a switch.

All IP multicasts are sent in Ethernet frames whose destination MAC address is a synthetic MAC address derived from a certain MAC prefix and the destination IP address (an address of a multicast group). These destination MAC addresses are never used as addresses of senders, and so they cannot be learned by a switch because the switch learns about attached hosts using only the source MAC address field. Consequently, to a switch, every frame carrying a multicast IP packet is a frame sent to an unknown destination, and so the switch floods the frame through all remaining ports in the same VLAN, just like it would do with any other frame with an unknown destination MAC address.

So - the switch simply floods all received multicasts because the destination MAC address is set to a value the switch can never learn, as the sender always uses its own MAC address to send multicast frames.

The router doesn't send an IGMP join does it?

No, it doesn't.

Should the L2 switch forward all multicast traffic toward the multicast router? 

Yes, actually, it will flood it through all ports in the corresponding VLAN except the ingress port. A switch running IGMP Snooping may optimize the flooding but I suppose we're talking about a basic, potentially unmanaged switch here.

Best regards,
Peter

[ben.wiechman]

Technically we're talking a 2960G, which is forwarding, and a Calix E7-2, which does not appear to do so. 

In the case where the switch is running IGMP snooping what changes with the behavior? Snooping of general membership queries or static configuration identifies the multicast router? 

[Joseph W. Doherty · Accepted solution]

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/10559-22.html#igmp_snooping

[ben.wiechman]

Thank you. That provides the clarification I needed.