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Comment by nine_k | Hacker News Reader

RPi2 had an Ethernet adapter internally connected via USB2. Probably this is not what you want for a cluster anyway.

Many competitively-priced ARM boards with proper Ethernet exist.

Yes, but having a proper ethernet on the edge connector would avoid the need for an RJ-11 and allow integrating a switch on the board hosting the CMs and prevent the usual mess with cables forming around the boards.

At least, that's what I'd hope.

The Pi has a single USB OTG interface, and no onboard ethernet. On the regular Pis, that's sent to an on-board hub, which fans it out to the USB sockets and the USB-Ethernet chips. The Compute Module, on the other hand, doesn't include a hub and Ethernet chip, because not every application will need it, and probably more importantly, it can be set to boot off code it receives from a USB host. This is useful in debugging because you can start working on things with a very minimal number of connections, and also so you can rapidly reprogram it without having to reflash for every boot.

In general, the CM has a lot of support for I/O that is just not feasible with the package the regular PI uses. Things like the extra camera and display ports can be useful for computer vision packages, but would make routing on the normal board real difficult. By minimizing the extent of the breakout, they really do make the chip able to be considered in a lot more use cases.

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