The problem was the following.

Run oaisim with two UEs:

    sudo ../cmake_targets/oaisim_build_oai/build/oaisim -O enb.conf -u2 -s15 -AAWGN -y1 -b1 -Q0

Then wait for both UEs to be connected. The interfaces "oip1" and "oip2" are
activated, with IP addresses 192.172.0.2 for the first UE and 192.172.0.3 for
the second.

Then on the EPC machine, do:

    ping 192.172.0.3

No reply.
On the oaisim machine, we see that the packets are sent to "oip1" instead
of "oip2".

The fix may break softmodem UE. The inst was forced to "1" for some reason.
To be checked.