Debian :: IP Packet Routing

I have recently upgraded a computer from Wheezy to Jessie, and I'm having trouble getting an internet connection shared via Ethernet by another computer (the provider) to work on it.I have activated the interface of the Jessie computer and configured a static IP on it in the same subnetwork as the provider's ethernet interface with the following commands:

Code: Select all# ip addr add 192.168.123.201/24 dev eth0
# ip link set up dev eth0

I now would like to set the address of the provider as the default route with Code: Select all# ip route add default via 192.168.123.100 dev eth0, but I get the following error message: Code: Select allRTNETLINK answers: File exists.

Indeed, when I run Code: Select all# ip route, the following comes up:

Code: Select alldefault dev eth0  scope link

I've tried to remove this default route to replace it with mine with Code: Select all# ip route flush table main and Code: Select all# ip route del default but these commands don't seem to work.