In a dumbbell topology, packets are not routable from CSMA nodes to the
interfaces at the far end of the connecting point-to-point link.  Routing works
to the nodes on the stub networks at both ends.  Problem happens when the echo
client is on the same node as one end of the point-to-point link and is echoing
to a node on the CMSA on the far side of that point-to-point link.  No route is
found for the point-to-point network on the return trip.

Reported by Mike MacGregor.