I am unable to ping from a samr21-xpro border router to my Ubuntu 16.04 workstation. I use slip and the tunslip tool for serial communication. See the gcoap Makefile.slip for setup. I know I need to move on to ethos, but I would expect slip to be working. Have I missed something?

You can see README-slip.md for the procedure. I'm leaving out RPL for this test, so I just do the steps below. The board is bbbb::2, and the tun interface is bbbb::1.

$ make term BOARD="samr21-xpro" PORT="/dev/ttyACM0"
>ifconfig 8 add unicast bbbb::2/64
>ncache add 8 bbbb::1
>ping6 bbbb::1

I did a little experimenting to find a good build.