Hans Bulfone wrote:
> 
>  -----------+------------ LAN: a.a.a.56/29
>             |
>             | a.a.a.57
>          NetBSD/alpha box
>             | a.a.a.61
>             |
>             .
>             . ethernet bridge
>             .
>             | b.b.b.17
>         Linux router (my default gateway)
[...]
> Have I missed something or is such a setup impossible ?

Well, as far as my understanding of IP routing goes, this
setup is impossible. There might be a chance to use
proxy-ARP for this, but I don't know if this would work, either.
You could try an interface alias b.b.b.16 on the 
netbsd a.a.a.61 interface.

  Guenther