Your company (Company ABC) makes a new contract with a Partner for a new research project. This requires connectivity between the Research Departments of the two companies.
Unfortunately, both companies uses the same private addressing space,, for internet network.
In order to perform the connectivity, Network Address Translation needs to be configured, but your partner company does not have a dedicated network team (they outsource the network changes to external parties, when needed) so in order to make this work as soon as possible, the NAT is performed on Company ABC's border router (R2) as shown below:


The target is:

  • your company's server will be seen as on the partner side
  • partner server will be seen as inside your company's network

Another constraint that exist within your network, is the fact that your border router (R2) is already using the old-style NAT (as apposed to new NVI, NAT Virtual Interface, feature) to hide/translate your internal network behind serial0/0 interface towards the Internet (R5=ISP), so for the moment, you have to use the legacy methods.

Unfortunately, there's something wrong with the new NAT because the connectivity between R1 and R4 is not working.
Here is the troubleshooting that you perform (when pinging from towards

R2#deb ip nat detailed
IP NAT detailed debugging is on
*Mar  1 01:26:24.335: NAT*: o: icmp (, 3) -> (, 3) [16]
*Mar  1 01:26:24.335: NAT*: s=>, d= [16]
*Mar  1 01:26:24.335: NAT*: s=, d=> [16]
R2#sh ip nat translations
Pro Inside global      Inside local       Outside local      Outside global
--- ---                ---      
---       ---                ---
R2#un all

What is the problem?

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