Hey Mark! > 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?
Selecting the proper interface for sending, is task of the network layer and is implemented according to the procedures explained in the according RFCs  in GNRC. This means that a node will first check if the destination prefix is "on-link" (if applicable) and use the according interface. If not, it will look up the destination in the FIB (Forwarding Information Base) if available (e.g., if a routing protocol is available or someone else filled the FIB manually) for the next hop's IP address. You can check the content of the FIB using the `fibroute` shell command. Finally, the neighbor cache (check with `ncache` shell command) is checked or a neighbor solicitation may be performed. Hence, if your IPv6 prefixes are configured correctly (e.g. as described in the gnrc_border_router example's README), the appropriate interface should be chosen. > 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. The border router is (as the name suggests ;-)) a router, not a bridge. Forwarding packets via the correct interface is done according to the rules stated above. HTH, Oleg  https://tools.ietf.org/html/rfc4861#section-5.2  https://tools.ietf.org/html/rfc6775#section-5.6 -- I'd make a joke about UDP, but I don't know if anyone's actually listening...
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