Years ago I was faced with an interesting challenge: an Internet customer was connected to our PE router with an Ethernet link and I did not want to include the PE router’s IP address in the default route on the CE router.
The latest IOS release in those days was probably somewhere around 11.x; none of the DHCP goodies were available.
After pondering the problem for a while, I got a brilliant idea: if I would use an interface default route, proxy-ARP would solve all my problems. This is the configuration I’ve deployed on the CE-router:
interface Ethernet 0
description Uplink to the ISP
ip address 10.0.1.2 255.255.255.0
ip route 0.0.0.0 0.0.0.0 ethernet 0
We tested this configuration in the middle of the night and it worked as expected. What do you think happened in the morning?