From CCO:
You can configure simplex Ethernet interfaces. This feature is useful for setting up dynamic IP routing over a simplex circuit (a circuit that receives only or sends only). When a route is learned on a receive-only interface, the interface designated as the source of the route is converted to the interface you specify. When packets
are routed out this specified interface, they are sent to the IP address of the source of the routing update.
So, for example, we have this:
int e 0
transmit-interface ether 1
Any routes that are learned from ethernet 0
(a receive only interface) will show up
in the routing table as pointing to
ethernet 1; all traffic that is destined
for the remote routes learned from
ethernet 0 will be sent out with the
next hop address that was included in
the route update, but the packet transmission
will be on ethernet 1.
