Topology-defining card units are used to provide optical interconnections between multiple slots of an equipment subrack. An example card unit is adapted for installation in a slot of an equipment subrack having a plurality of slots and having a backplane. The card unit includes one or more back-side optical connectors configured so as to mate with corresponding optical connector receptacles on the backplane of the equipment subrack when the card unit is installed in the equipment subrack. These one or more back-side optical connectors include a plurality of card-unit optical interfaces. The card unit further includes an optical interconnection network that optically couples each one of the plurality of card-unit optical interfaces to another one of the plurality of card-unit optical interfaces.