To set a static ARP entry, the command used to be arp -s IP MAC, but on Server 2008 R2 and perhaps others back to Windows 7/Server 2008, this command may not work even if Run as Admin. However, the netsh commands in this Q&A provide alternate syntax that does work for the same purpose.
"The Managed Switch Port Mapping tool is designed to communicate with a managed ethernet switch using SNMPv1/v2c and map the physical port connections to MAC and IP addresses of the attached devices. The software is designed for use on Microsoft Windows® operating systems."