I am relatively new to RIOT and I hope someone can provide some expertise
here; I am exploring the gnrc networking stack in the pursuit of a
functioning border router.

This question is centered around "examples/posix_sockets."  I am using
ATMEL SAMR21 chips.

The posix_example works as expected, without modification, provided I am
communicating between two SAMR21 nodes using the 802.15.4 radio (SAMR21
depends on at86rf2xx).

BUT, I am also working towards a functioning border router by adding an
ethernet interface to the SAM.

I have explored the edge case (no pun intended) of the posix_sockets
example where I use ONLY an ethernet adapter (i.e., replacing the
dependency on at86rf2xx with ksz8851snl, the ethernet driver).  It works as
expected: I am able to send UDP data from the SAM over the ethernet to my
computer's UDP server (and back).  In this edge case, the ethernet is the
only interface the SAM reports, also expected.

But, when I combine these two cases -- so that I have both at86rf2xx and
ksz8851snl interfaces enabled -- it seems that UDP data is only transmitted
via the radio!  Other SAMs can receive the UDP packets, but the computer
server stops receiving them.  No IP addresses were changed; only the number
of interfaces available to the SAM (i.e. GNRC_NETIF_NUMOF := 2).

This leads me to a slightly deeper question -- how does RIOT decide what
physical interface to attempt to communicate over when I use network send
calls such as conn_udp_sendto?

As an associated question -- is it the intention of the gnrc_border_router
example to automatically bridge interfaces (such as ethernet & 802.15.4)?
As far as I can tell it really only initializes interfaces but does not
provide any actual routing between them.

Thank you,

Mark Solters
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