An optical network device re-routes traffic from a path to a backup path in response to determining that a downstream segment of the primary path is not operational. The optical network device receives traffic on a slot of an optical fiber. For each data unit in the traffic, the optical network device determines, based on receiving the data unit on the slot and based on a flow identifier specified in the data unit, that a given path is associated with the data unit. If a downstream segment of the given path is not operational, the optical network device routes the data unit onto a backup path instead of routing the data unit along the given path. Bandwidth is not reserved for the backup path.