ARP resolution between two XMOS devices?

[jamesd]

I'm in the early days of prototyping a network application for XMOS devices. Since I have access to two devices, I decided to flash each with an app I coded that is intended to:

a) listen for packets on a given port, and light LEDs when they show up
b) at the press of a button, emit a TCP packet aimed at the other device, which will then do a)

Each device is configured to use a static IP and has the other device's static IP coded as a target address. (I'm using xtcp 1v4.) So, for instance, box A is configured as 10.0.0.251 and box B is configured as 10.0.0.253.

Part a) works nicely: if I send traffic to either box from my workstation I get the expected response(s). So far, so good.

Part b) doesn't want to work. Using Wireshark, I can see that if I trigger a packet from either device, the device issues a broadcast ARP request trying to resolve the target address, which never gets answered.

If I ping either device from my PC, the PC broadcasts an ARP request which prompts an immediate response from the device and allows ICMP to proceed.

Does this sound familiar to anyone? I suspect I'm doing something dumb, but a dose of the flu hasn't been helping with diagnostics. Thanks!