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  1. Does the segment get delivered -- and the recipients are supposed to drop it?
  2. Does the segment get dropped at the (layer 3) switch/router?
  3. Or something completely different?

Explanations or "RTFM!" comments (which include RFCs + section hints) are highly appreciated.

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In an Ethernet network a TCP segment isn't delivered to any address. TCP isn't concerned with source or destination ip addresses. That's the job of IP. In an Ethernet network a TCP segment is encapsulated in an IP packet, which is in turn encapsulated in an Ethernet frame. For an IP packet with a subnet broadcast destination address the frame is delivered to all hosts connected to the same physical network segment (this is because a layer 3 broadcast has the same layer 2 destination address as a layer 2 broadcast (FF-FF-FF-FF-FF-FF). All hosts with ip addresses in the same Layer 3 network will "consume" and process the packet. All hosts not in the same Layer 3 network will "consume" the packet, determine that it isn't for them, and discard it.

For a Layer 2 broadcast (where the Layer 3 destination address is 255.255.255.255 and the Layer 2 destination address is FF-FF-FF-FF-FF-FF) all hosts connected to the same physical network segment will consume and process the frame.

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  • Thank you for the detailed answer. The "reason" of/behind my question was: "Layer 3.5" switches/routers provide NAPT/PAT (which, as the labelling suggests, combine some of layer 3 and layer 4 functionality). So -- out of curiosity -- ...why forward TCP "broadcast" segments (which clearly -- if I'm not mistaken -- makes little sense)?
    – fbahr
    Dec 14, 2018 at 17:59
  • Most of the devices do not concern themself with policy decisions on a layer they are not responsible. This includes filtering based o protocol/addresstype. Some do it as they claim to have security features. If the device has to do PAT(stateful NAT they have to implement some TCP level logic and are then more likely to do more validation. Having said that you can of course set up an ACL.
    – eckes
    Dec 17, 2018 at 3:54

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