[darrin]

They're more like digital patch panels. You can connect any pair of ports together in both directions (TX/RX), just like moving a patch cable around. You can also connect an incoming RX lane to multiple TX lanes (think optical splitter but electrical). You cannot merge signals together. Some products have FPGAs that add L2+ or multiplexing functionality with packet buffers, but that's not part of the 2-5ns path.

[brewmarche]

So, if I know the destination addresses on each port in advance I can program it to behave like an L2 switch (with proper multicast even), and I can use code to reprogram it whenever the traffic pattern changes?

[darrin]

The L1 path can't do any address matching and you can't overlap sources - like Ghostbusters, no crossing the streams. You can source port 1 to ports 2-4 (multicast, but static and unidirectional) and you can change a bidir connection between pairs of ports. The signal gets replicated regardless of what's in the frame. It wouldn't be practical to make it behave quite like an L2 switch.