After changing the number of available CPUs in the Hyper-V Console, the Messaging Gateway (SMG) loses network connectivity.
Setting the number of available CPUs to 1 returns network connectivity to SMG but at a significantly reduced performance level due to only a single CPU being assigned to the virtual machine.
This is a known issue with the Broadcom NetXtreme gigabit network adapter on Windows Server 2012 and 2012 R2 using the b57nd60a.sys driver.
This issue may be resolved by updating the Broadcom NIC driver as described in Microsoft KB Virtual machines lose network connectivity when you use Broadcom NetXtreme 1-gigabit network adapters
Setting the virtual machine network interfaces (NICs) in Hyper-V to "Legacy Network Adapter" rather than the default "Network Adapter" appears to resolve this issue.