Comment 23 for bug 438454

Furthermore, if the Network Connections applet is used to change the IP or some other IPv4 parameter, any previously-set default route is deleted: (see comment #22 etc.)

Before using the applet:

peter@lightning:~$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.2.0 * 255.255.255.0 U 1 0 0 eth0
192.168.1.0 linux.router 255.255.255.0 UG 2 0 0 eth0
link-local * 255.255.0.0 U 1000 0 0 eth0
default cisco831 0.0.0.0 UG 66 0 0 eth0

After:

peter@lightning:~$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.2.0 * 255.255.255.0 U 1 0 0 eth0
192.168.1.0 linux.router 255.255.255.0 UG 2 0 0 eth0
link-local * 255.255.0.0 U 1000 0 0 eth0
peter@lightning:~$

The route through the Linux router (192.168.2.201) is kept, but the default route through the Cisco router is not.