Making sure a particular NIC is considered EtherX

Please forgive me if this is covered elsewhere, but I searched and could’nt find an answer.

I have a XEN VM running RouterOS 3.14. It has two NICs and periodically when I reboot this router VM the ‘etherX’ designator flips from one NIC to the other. How can I make sure a “NIC1” will always be considered “Ether1”?

Thanks.