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I set up a port based VLAN on the first few ports of a 10GbE switch for broadcasting a UDP data stream. The data stream should use maybe 1/4 of that bandwidth. The switch is a netgear M7100.

The problem is I am losing about 25% of the packets when broadcasting. I've tested it with unicast on the VLAN and didn't lose anything.

I've wired two server's NICs together directly and sent the stream to the broadcast address and do not lose a single packet either.

I've looked at wireshark and I dont see anything that pops out. It shows it sending all the packets but the receiver only sees 3/4 of them.

I've increased the kernel network buffer sizes in sysctl.conf and it didn't make much difference.

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  • Look at the interface counters on the switch. 2.5Gbps of broadcast traffic will almost certainly trip storm-control. Also, the switch has a single 16MB buffer shared across all ("active", so they say) ports.
    – Ricky
    Mar 12 '15 at 22:39
  • @Ricky Thank you! Storm control was the issue! First time setting up a managed switch :). I'll mark it as solved if you put the answer up
    – Nate
    Mar 12 '15 at 22:57
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    Please consider reading multicast vs broadcast in a LAN. 2.5Gbps of broadcast is insane Mar 13 '15 at 8:44
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    Also note, multicast is a special case of broadcast. As such, dumber switches (esp. unmanaged ones) make no distinction between them. In wireless (802.11), multicast is normally processed exactly like broadcast -- no pruning, sent to all radios, at basic rate [one frame, every radio has to be able to receive it] (special handling can do multicast to "unicast" conversion. this is, in theory, how uverse wireless receivers work and why you must use their wifi gateway)
    – Ricky
    Mar 13 '15 at 20:37
  • ("unicast" because it doesn't actually change the frame, just how it's processed. it's sent the same as unicast directly to an associated client.)
    – Ricky
    Mar 13 '15 at 20:38
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"1/4 of that bandwidth" would be 2.5Gbps. That's a lot of broadcast traffic. Managed switches have a safety system to prevent broadcast floods ("storm-control".) First guess, you've exceeded the storm-control limit by many orders of magnitude.

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