When does Multicast Duplicate?

[KGrev]

Hi,

I just have a quick question. In this scenario, at which points does multicast copy/duplicate the packets to send to the host in the group? Does it replicate at switch 1 and 2? Or since SW3 is only trunked to SW2 and how no layer 3 interfaces, does SW2 do the replicated and send that packets across the trunk?

Thank you for your help.

[Harold Ritter]

Hi @KGrev ,

SW2 is the last hop router (L3) and is the one receiving the IGMP membership reports from the workstations. It is therefore the device performing the replication to the various output interfaces.

Regards,

[KGrev]

@Harold RitterThanks for your response. So then if SW2 is doing the replication, is it sending 5 (example) replicated packets across its trunk? Meaning, is the bandwidth usage on the trunk between SW2 and SW3 higher than if it was just one stream?

[Harold Ritter]

Hi @KGrev ,

Only one copy of a multicast group traffic is forwarded to SW3 per VLAN. I stated that SW2 would perform the replication, but that was assuming that the workstations were attached to separate VLANs. If they are all part the of the same VLAN then only one multicast stream gets forwarded between SW2 and SW3.

Regards,

[Joseph W. Doherty]

Unless IGMP snooping is not supported or not enabled, each switch will replicate any multicast packet to all ports in the same L2 broadcast domain (basically the same as would be done for a broadcast packet).

If IGMP snooping is active, switch will only replicate multicast packets to ports that "desire" particular multicast packets.

BTW, since you have a RP and PIM interface, on switches, that implies they are L3 switches.  Multicast routing can, in some topologies, "needlessly" replicate multicast packets between multicast routers, but Cisco's PIM snooping addresses that issue.