How to set up Cisco NAT for multiple routers, where one is connected to the internet

Question

Two Routers:
Router A:

1. F0/0 with a public IP to the internet: 12.12.12.12
2. F1/0 which is a LAN: 10.10.10.1
3. S0 which is one of a point to point connection: 192.168.1.1

Router B:

1. S1 which is the other end of the point to point 192.168.1.2
2. F2/0 which is the LAN: 10.10.20.1

How can I set up nat (overloading) so that traffic from router B (The 10.10.20.1 Lan) can use the F0/0 internet connection on router A?

Is it all done on router A?

I would think it works like this: I just set F0/0 to nat outside, and S0 to nat inside, and add the 192.168.1.* network to the ACL for the nat overload rule? But I never done this set up before...

Answer 1

You're on the right track. Yes, it is all done on Router A. You will want both F1/0 and S0 to have ip nat inside on them. The ACL on Router A's ip nat inside source command should include both 10.10.10.x and one of the following:

1. If Router B is doing NAT on its own, so Router A only ever sees traffic from 192.168.1.2, you need to add 192.168.1.2 (or 192.168.1.0/30) to the ACL.
2. If Router B is not doing NAT, (which it probably isn't), then you need to add 10.10.20.x to the ACL (and probably also 192.168.1.2 if Router B needs to talk to the Internet).

Basically, the ACL just needs to cover any source IP you want to NAT as it appears to Router A when it enters an interface with ip nat inside on it. Traffic from the Router B LAN will be seen by Router A as 10.10.20.x (unless Router B does its own NAT overload).