Cisco NAT for exchange 2010

Posted on 2011-09-19
Last Modified: 2012-05-12
All, I have a cisco 1800 series router that I need to have an exchange server behind.  It's running IOS without the firewall enabled, rather, access is more or less turned off or on on a one by one basis.

I have an exchange server with the proper MX and A records at the ISP level.  The current configuration worked previously by doing a NAT translation inside and out like this:

ip nat pool exchange netmask
ip nat inside source static network /32
ip nat outside source static network /32

This configuration worked with the Cisco 831 for over a year.  We did get an occasional IP conflict error when we had an outage and came back up, but I was always able to program around it by changing the NAT to another internal IP, then bringing up the server, then changing it back.  I realize this was a redneck solution, but it did work and I figured I'd figure out the issue later.

Later is now.

When we replaced the 831with a cisco 1800, it reports an IP conflict. at both the router, and server.  The NAT translation is using the address and the exchange server shows as 'no internet access' as you would expect from an IP conflict.  It is not resolveable by the workaround I previously mentioned.  The newer router is far less tolerant, it seems, of this kind of NAT/IP conflict.

I have no idea how it worked before given what I now know.  However, I'm not at all sure how to fix this going forward.  what is the correct way to make this work, using the IP information above?  I need to be sure that OWA access is available inside and outside the internal network, and that direct access through outlook is available internally.  Help!
Question by:John W
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Expert Comment

ID: 36564372
You don't need the nat pool for a static NAT.  Just the statement below it
LVL 35

Expert Comment

by:Ernie Beek
ID: 36565765
The only statement you need is: ip nat inside source static network /32

The other two just mess things up ;)
LVL 35

Accepted Solution

Ernie Beek earned 300 total points
ID: 36565782
After re-reading, also remove 'network'. So then you get:
ip nat inside source static

One other question, is the public ip the address of the outside interface or an extra one?
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Author Comment

by:John W
ID: 36596839
Sorry for the belated response.

The IP is not the same as the external interface, rather, an extra one as part of several we have.  DNS, MX records, etc are assigned to it to create / and so forth.
LVL 35

Expert Comment

by:Ernie Beek
ID: 36709808
Ok, then it should be like this.

Remove the three lines:
ip nat pool exchange netmask
ip nat inside source static network /32
ip nat outside source static network /32

And replace them with:
p nat inside source static

Then let's see where that leads you.

Author Closing Comment

by:John W
ID: 36712052
Thanks so much!
LVL 35

Expert Comment

by:Ernie Beek
ID: 36712124
You're welcome :)
Thx for the points.

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