Final answer:
The Cisco IOS command to assign a static IPv6 address using an EUI-64 interface ID is 'ipv6 address' followed by the network prefix and the keyword 'eui-64'.
Step-by-step explanation:
The command used in Cisco IOS to assign a static IPv6 address using an EUI-64 interface ID is ipv6 address followed by the IPv6 prefix and the keyword eui-64. For example, if you are configuring an interface with an IPv6 network of 2001:db8:1234:5678::/64, the command would be ipv6 address 2001:db8:1234:5678::/64 eui-64. This commands tells the router to generate the last 64 bits of the IPv6 address based on the interface's MAC address, creating a unique IPv6 address that incorporates EUI-64 format.