Hi there !
Following up the "routed interfaces" thread, could 1812 owners please try if the following it's possible to configure
interface FastEthernet1 no switchport ip address ...
and then do routing between FE0 and FE1 or even between FE1 and FE2? On an
1841 with HWIC-4ESW, it should work (with FE2 and up).
TIA
fw