How to terminate nested ssh automatically?

I'm using a nested ssh such as the following to port forward between three
machines. This forwards the port but the nested ssh (running on ud00) does
not terminate when I terminate the original command. Is there a better way
so that I don't leave an orphaned ssh process?

ssh -L9000:localhost:9000 ud00 'ssh -N -L9000:10.162.13.143:7710 ud01'

Same version on all machines.
$ ssh -V
OpenSSH_5.0p1, OpenSSL 0.9.8g 19 Oct 2007
$

John