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I'm trying to test UDP multicast throughput via iperf but it seems like it's not sending more than 10Mbit/s from my dev machine:

C:\> iperf -c 224.0.166.111 -u -T 1 -t 100 -i 1 -b 1000000000
------------------------------------------------------------
Client connecting to 224.0.166.111, UDP port 5001
Sending 1470 byte datagrams
Setting multicast TTL to 1
UDP buffer size: 8.00 KByte (default)
------------------------------------------------------------
[156] local 192.168.1.99 port 49693 connected with 224.0.166.111 port 5001
[ ID] Interval       Transfer     Bandwidth
[156]  0.0- 1.0 sec  1.22 MBytes  10.2 Mbits/sec
[156]  1.0- 2.0 sec  1.14 MBytes  9.57 Mbits/sec
[156]  2.0- 3.0 sec  1.14 MBytes  9.55 Mbits/sec
[156]  3.0- 4.0 sec  1.14 MBytes  9.56 Mbits/sec
[156]  4.0- 5.0 sec  1.14 MBytes  9.56 Mbits/sec
[156]  5.0- 6.0 sec  1.15 MBytes  9.62 Mbits/sec
[156]  6.0- 7.0 sec  1.14 MBytes  9.53 Mbits/sec

When I run it on another server, I'm getting ~80Mbit/s which is quite a bit better but still not anywhere near the 1Gbps limits that I should be getting?

C:\> iperf -c 224.0.166.111 -u -T 1 -t 100 -i 1 -b 1000000000
------------------------------------------------------------
Client connecting to 224.0.166.111, UDP port 5001
Sending 1470 byte datagrams
Setting multicast TTL to 1
UDP buffer size: 8.00 KByte (default)
------------------------------------------------------------
[180] local 10.0.101.102 port 51559 connected with 224.0.166.111 port 5001
[ ID] Interval       Transfer     Bandwidth
[180]  0.0- 1.0 sec  8.60 MBytes  72.1 Mbits/sec
[180]  1.0- 2.0 sec  8.73 MBytes  73.2 Mbits/sec
[180]  2.0- 3.0 sec  8.76 MBytes  73.5 Mbits/sec
[180]  3.0- 4.0 sec  9.58 MBytes  80.3 Mbits/sec
[180]  4.0- 5.0 sec  9.95 MBytes  83.4 Mbits/sec
[180]  5.0- 6.0 sec  10.5 MBytes  87.9 Mbits/sec
[180]  6.0- 7.0 sec  10.9 MBytes  91.1 Mbits/sec
[180]  7.0- 8.0 sec  11.2 MBytes  94.0 Mbits/sec

Anybody has any idea why this is not achieving close to link limits (1Gbps)?

Thanks,

Tom

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Multicast would be the first place to look, do you get the same limits for unicast TCP testing? – polynomial Oct 5 '11 at 1:09
Are you connected to a switch that supports pause frames? If so, broadcast and multicast will be limited to the speed of the slowest port. – David Schwartz Oct 5 '11 at 2:10
hm, I do have an IP Phone plugged into the same switch on the same subnet which is probably running at 10MBit - could that be the limiting factor? However, the switch on the server is a Cisco 3750G, fully running at 1Gbit on all links, why wouldn't I manage to achieve more then 10% of link capacity there? – Tom Frey Oct 5 '11 at 2:36
I turned off Flow Control on the NIC of my dev machine and am now getting ~10MBytes/sec, same as on the server. So that definitely helped but is still only 10% of link capacity, any idea why? I tried adjusting the UDP send buffer which didn't seem to have any effect – Tom Frey Oct 5 '11 at 13:53
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are the any multicast or broadcast storm protections enabled on the switch. – The Unix Janitor Feb 1 at 11:10
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Check if your switch (your network cards should already support it) is accidentally not set to auto-mdix or if there is fixed value and you got a mismatch. This can cause the problems you describe, I've seen the exact same behaviour in our own network with a mismatch. Sometimes auto-mdix might not work and you should also try setting it to the same value on both sides manually.

http://en.wikipedia.org/wiki/Mdix#MDIX

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