Tuesday, February 14, 2006


Route maps with static NAT. (Question #10)

The command to configure a static NAT entry also allows you to specify a route map. For instance, you could have

ip nat inside source static route-map TEST

The need for a route map for dynamic NAT is obvious. But when would a route map be useful for static NAT entries where the source address to be translated has already been explicitly specified?

I've had a case before where the source IP would always physically be the same IP (multiple users on one system), but based on its DESTINATION port, you had to NAT to seperate source IP, and change the destination port to the real destination port.

For instance: A server is running Terminal Servies and has 5 users on it, all running the same legacy telnet app. Each user is assigned a seperate DESTINATION port so we can seperate the streams, but its all destined for a telnet app on the other end.

The destination app doesn't agree with TS apps since each connection is coming from the same source IP and its not coded to handle this. To fix this issue, one could use Route Maps - identify the destination port, give it a different source IP via NAT (so each user from one physical system gets a differant source IP), keep the same destination IP, but change the destination port to 23.

This makes the remote app think that each session is coming from a seperate IP and that each IP's destination is port 23.

So there are some odd uses for it out there, but its hard to think of them unless you've actually be challenged with this situation.

A much simpler reason that I totally overlooked is if you've got multiple outbound interfaces, all with 'ip nat outside' specified. You'd want to control which address your source gets NATed to based on its destination. A little simpler than my previous comment :)

The previous scenario created a VERY complex route-map scenario not typically found in most environments.

Yes, I had something like your second scenario in mind. The first scenario did provide some food for thought though!
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