I used to connect my ubuntu 14.04 laptop on office network via eth0 and the route table gets updated with the office network gateway ip in auto mode.

Now, i changed my /etc/network/interfaces configuraton to add a bridge interface, br0 and made eth0 as a dumb port inside br0. My /etc/network/interfaces has,

iface br0 inet static
   address X.X.X.X
   netmask 255.255.0.0
   bridge_ports eth0
   bridge_stp on
   bridge_fd 0
   gateway X.X.X.X
 

No longer, my route table is getting updated. And everytime, i plug my ethernet cable, i need to do,

route add default gw X.X.X.X br0

How to configure the default gateway for a bridge?