ifup eth0 failed to bring up eth0

First, I have set static ip using the below commands: Commands:

ifdown eth0
ifconfig eth0 X.X.X.X netmask 255.255.252.0 up
route add default gw X.X.X.X

I was successful in setting up static ip X.X.X.X and I could see the same in the output of command "ifconfig".

Now I am trying to revert network back to dhcp using the below commands: Commands:

ifdown eth0
ifup eth0

Output :

RTNETLINK answers: File exists
ssh stop/waiting
ssh start/running, process 1524

ifup eth0, failed to bring back dhcp.

Contents of /etc/network/interfaces

root@bdhcp396:~# cat /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

Is this a bug in Ubuntu 11.10/10.04.3? I see a similar bug raised - https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/876829