Use the command
ifconfig bge0 mtu 1500
and NOT
ifconfig bge0 192.168.0.1 mtu 1500
ias explained in http://www.cyberciti.biz/faq/freebsd-jumbo-frames-networking-configration/ if trying to chage the MTU remotely via SSH. If you specifiy a a IP address FreeBSD seems to “reset the stack” and loses the default gateway