Cyclic Queuing and Forwarding (CQF) is a mechanism defined by IEEE TSN for providing low jitter in a deterministic network. CQF uses a common time cycle and two buffers per node output port. During one cycle, incoming packets are stored in one buffer while packets in the other buffer are being transmitted; at the end of a cycle, the roles of the two buffers are exchanged. CQF provides very simple bounds on latency and jitter. Its correct operation requires a large enough cycle duration so that all packets received by a node in one cycle can be forwarded during the next cycle. We give a necessary and sufficient condition for this to hold. Our condition depends on the link properties and the flow characteristics at the network input. We apply the condition to obtain the minimal admissible cycle duration. We observe that the minimal cycle duration is not always margin-safe, i.e., larger values might be nonadmissible, which suggests replacing the minimal cycle duration with the minimal margin-safe cycle duration.