Data flow in the context of collision and broadcast domains focuses on how data frames propagate through a network. It refers to the movement of data through Layer 1, 2 and 3 devices and how data must be encapsulated to effectively make that journey. Remember that data is encapsulated at the network layer with an IP source and destination address, and at the data-link layer with a MAC source and destination address.

A good rule to follow is that a Layer 1 device always forwards the frame, while a Layer 2 device wants to forward the frame. In other words, a Layer 2 device will forward the frame unless something prevents it from doing so. A Layer 3 device will not forward the frame unless it has to. Using this rule will help identify how data flows through a network. 

Layer 1 devices do no filtering, so everything that is received is passed on to the next segment. The frame is simply regenerated and retimed and thus returned to its original transmission quality. Any segments connected by Layer 1 devices are part of the same domain, both collision and broadcast.

Layer 2 devices filter data frames based on the destination MAC address. A frame is forwarded if it is going to an unknown destination outside the collision domain. The frame will also be forwarded if it is a broadcast, multicast, or a unicast going outside of the local collision domain. The only time that a frame is not forwarded is when the Layer 2 device finds that the sending host and the receiving host are in the same collision domain. A Layer 2 device, such as a bridge, creates multiple collision domains but maintains only one broadcast domain.

Layer 3 devices filter data packets based on IP destination address. The only way that a packet will be forwarded is if its destination IP address is outside of the broadcast domain and the router has an identified location to send the packet. A Layer 3 device creates multiple collision and broadcast domains.

Data flow through a routed IP based network, involves data moving across traffic management devices at Layers 1, 2, and 3 of the OSI model. Layer 1 is used for transmission across the physical media, Layer 2 for collision domain management, and Layer 3 for broadcast domain management.