Access Cisco Firewall forwarded external IPv4 port from inside

For some time now I am using a borrowed Cisco 881 router as router/firewall for my internet connection. The box is stable and configured as I want, but unlike with the Linux and Fritz!Box routers I used before, the Cisco does not allow to connect to forwarded IPv4 ports on its external address. This is inconvenient in my situation as this means that I am unable to reach some services from my internal network (i.e. I cannot reach websites I host). So far the only way around this was using split DNS and double administration, which is quite tedious and inconvenient.

Some time ago when looking how to set this up, I bumped into this article:  NAT: access outside global address from the inside (this site seems to be down at the moment, but it’s content is still available through here thanks to the Internet Archive). This describes an alternative way to setup the Cisco NAT rules using the NAT Virtual Interface (NVI),which decouples them from the specific interface in a specific direction. Today I have tested this approach.


To setup the new NAT approach, change the existing NAT rules:

ip nat inside source static tcp 80 WW.XX.YY.ZZ 80

into something that looks like the next line:

ip nat source static tcp 80 WW.XX.YY.ZZ 80

ip access-list extended NAT-INSIDE-ADDRESSES
permit ip any
ip nat source list NAT-INSIDE-ADDRESSES interface FastEthernet0/1 overload

(basically remove the inside clause in the statement). In my setup is the internal IP address of my web server and WW.XX.YY.ZZ represents my external IP address. In this example I forwarded port 80 (HTTP). The last part is required to make sure that also internal traffic on FastEthernet0/1 will be NATted properly to avoid asynchronous data flows.

 Testing it

The first basis tests of this new setup were promising. Indeed, after these changes I could access my external sites also from internal addresses. However, when downloading something from an internal site I noticed that the performance was not very good. This was something I definitely could live with as the traffic would not be massive. However, due to this change in config, all NAT traffic turned out to be slower and effectively the performance of my network connection was about half of what it used to be. Before this change the Cisco 881 was capable of streaming about 38 – 43 Mbit, which was not my full 50Mbit bandwith, but close enough. With this (NVI) setup, I noticed that my max. network bandwith  using SpeedTest.NET dropped to 20Mbit and below. With the command
show processes cpu history
on the router I noticed that the poor Cisco 881 was at 100% CPU load/utilization during the downloads. I suspect that the old Cisco 881 (which does not support 50Mbit in the first place) is CPU-bound when using NAT Virtual Interfaces and not capable of handling this at higher speeds.


Technically, the approach to use the NAT Virtual Interface (NVI) feature of IOS works to enable access to NAT forwarded external ports from the inside. However, since this appears to be very CPU intensive, it is not a good solution for now as the Cisco 881 cannot cope with the load and the internet bandwith is effectively reduced to only 50%. I think need to revisit this approach once I have acquired a router that is capable to support the bandwith I have and see if then can handle the CPU load.

One Reply to “Access Cisco Firewall forwarded external IPv4 port from inside”

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.