I have a 2811 router where one of its 2 FE interfaces is used to connect to the ISP. We have quite a number of requirements for multiple IP addresses. These additional IP addresses were configured on loopback interfaces and are accessible from over the internet for the purposes they are needed.
We recently got a new ISP setup and also bought an additional 2 port FE card where the new ISP link would be terminated. While trying to setup the new ISP in the same manner as the previous one on the same router, I ran into a problem setting up the additional IPs in the same manner by using the loopback interface:
ROUTER(config-if)#ip add a.b.c.94 255.255.255.192
% a.b.c.64 overlaps with Loopback11
I didnt setup the initial ISP on the router, so I am unsure why the loopback interface was used. In my previous configurations, I setup using secondary IPs on the primary FE interface.
With this error stated above, I would like to understand the advantage of using the loopback interface as against the secondary IPs on the main interface as I need to resolve this issue and go ahead to setup the additional IPs from the new ISP for different uses within the office environment.
It is fairly simple to configure static address translation of an inside address to an outside address. The command looks like
ip nat inside source static
and the outside address does not need to be configured on an interface.
You do need ip nat inside on the inside interface and ip nat outside on the outside interface. But I assume that you probably already have these.
for details see this link