Currently, guest NICs are provided with random MAC addresses, with an Oxide OUI and within the "virtual MAC" range. That is, their first 3 bytes and a nibble are a8:40:25:F_:__:__. On the other hand, the OPTE virtual gateway will always be provided a MAC address as well, which must not conflict with that of the guest NICs. We currently ensure that the MAC is unique among guest NICs with a unique index on the table, but also need to add this new check.

See this thread for a discussion of the actual MAC chosen for the OPTE gateway.