network bandwidth discrepancy

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http://serverfault.com – I have a cluster of machines with two 10Gbps intel network cards on each machine. 64 port Router, with each port of capacity 10Gbps is being used. when i try to measure maximum network performance between machines connected to same router, I see that i get close to 99% bandwidth for a pair of network cards between only 2 specific machines, but i get only around 40-60% bandwidth between rest of pairs of machines. The driver settings are the same in all machines. The network test tools i used were iperf and NTttcp. All machines are identical with respect to CPU, memory, network cards. I trie (HowTos)