Having the same problem:
new R720
ESXi5.1
DGS-1100-24 DLINK switch
I350 NIC ports connected to the DLINK switch will drop connection to the switch. I updated BIOS, IDRAC, NIC, LC firmware to the latest found on Dell's website. Didn't help.
Update the igb driver to the latest available. Didn't help.
Anyone been able to resolve this issue?