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I have following issue:

I have unit A which is connected to a non configurable switch which is totally flat and this switch is connected to a switch which has capabilities of layer 3 (BATM T5CL3 G series).

This unit A is sending a multicast messages to 224.10.10.10 and also sending igmp join to 224.10.10.10.

There is a port monitor on the BATM switch where I've checked the TX buffer and couldn't saw any multicast sent back to unit A (but I can see on the RX buffer the generated multicast).

The BATM SW configuration is default one (no VLANS, no ACLs...) except igmp snooping and port monitor.

How can I set a configuration on a switch so that this generated multicast message (by unit A) would be received by unit A as well?

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    At layer-2, there is no protection from looping like a layer-3 TTL. You cannot send traffic back the way it came in. That risks creating a layer-2 loop, which will bring down the entire LAN. Such loops are very difficult to fix.
    – Ron Maupin ♦
    Oct 10, 2017 at 12:53
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Feb 19, 2018 at 19:34

1 Answer 1

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Multicasts are not normally sent back from where they came (otherwise they'd loop). An unmanaged switch - or a managed switch without IGMP snooping - treats multicasts as broadcasts.

As a - crude - workaround you could set up mirroring from another multicast subscribed port to the port your device is connected to. Beware that in your scenario where it's connected to the unmanaged switch this means you'd be doing it on the far switch, reflecting back all multicast packets. All clients on the unmanaged switch will receive all multicasts twice.

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  • I know that this is usually the status (Multicasts are not normally sent back from where they came), however, I need this for a particular scenario.
    – user1977050
    Oct 10, 2017 at 6:56
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    The local stack should take care of local multicast subscribers. If you loop back multicasts (somehow) you'd have a loop - I guess you could work something out with routing multicasts back to another segment and back again (on another switch/router), but how do you think you can stop multicasts to circle once they loop back? How about not sending them as multicast at first but as unicast to a UDP relay which in turn sends the multicast (back!)?
    – Zac67 ♦
    Oct 10, 2017 at 10:54

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