Unfortunately, problems have arisen with WS2012 R2 Hyper-V networking where Emulex NICs are used. Hans Vredevoort (Hyper-V MVP) and Marc van Eijk (Azure MVP) have been experiencing this issue and blogged about it on Hyper-V.nu. It appears that lots of you have encountered the same problem with VMs and virtual NICs losing connectivity when a virtual switch is connected to an Emulex NIC – in the guys’ case in HP blades with FlexFabric.
The core issue seems to be related to Emulex 10 GbE NICs – As they guys report in their post Kristian Nese (System Center MVP) also sees the problem on IBM servers with Emulex NICs.
There is no fix. Please watch the thread on Hyper-V.nu to keep up with updates.