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So a few questions that I've struggled to find firm answers on.

1: Does multicast use arp? So does it ask for a corresponding IP address?

I think the answer is no, as it translates the IP to a MAC itself. But I'd like confirmation of this please. Wireshark would also suggest it doesn't.

2: When a machine engages in multicast, what MAC address in the CAM table does the switch record for the machine? On an access port we can't allow 2 ports to have the same mac can we?

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Multicast is a version of broadcast, which sends the frames to all switch interfaces.

Having said that, there is something called IGMP snooping that allows switches that support this to listen in on the IGMP conversations between hosts and a multicast router to see which switch interfaces want traffic from specific multicast groups. Switches that support this will only send traffic destined for a multicast group to those interfaces that have a destination interested in the traffic destined to that multicast group.

Not all switches support IGMP snooping. It is not something that is required, and pure layer-2 switches will send multicast frames to all switch interfaces.


By the way, switches don't use ARP, which is a layer-3 to layer-2 resolution protocol. Layer-2 switches don't know anything about layer-3. Layer-2 switches do maintain MAC address tables to resolve layer-2 addresses to switch interfaces, but not to layer-3 addresses.

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  1. Multicast does not use ARP. Multicast IP Address mapped statically into Multicast MAC. Check here: https://networklessons.com/multicast/multicast-ip-address-to-mac-address-mapping/

  2. Source MAC address is recorded. On access port you can limit number of learnt MAC addresses to 1 (or any other number), or you can leave unlimited.

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