Problem with pinging from external server and ISP router.

[Friut]

Problem Summary

In a Packet Tracer simulation, the ISP router could not reach internal networks through RTR-1, even though traffic from the internal network (Host A) successfully communicated with the external server via NAT. Pings from the ISP to RTR-1's internal interfaces failed, suggesting issues with NAT, routing, or implicit filtering. We need help identifying why incoming traffic from the ISP is blocked while outgoing traffic works fine.

Link to Ufile with my PT 

[Flavio Miranda]

@Friut 

Attach the packettracer here directly.  You just need to zip the file first.

[Friut]

Hi, thanks was a bit confused how to do it. But should be there now

[Flavio Miranda · Accepted solution]

@Friut 

 you are missing one route on the ISP router. Your ISP router does not know how to reply.

[Friut]

Thank you so much!

[vishalbhandari]

@Friut Common Causes and Fixes

1. NAT Overload Missing ACL Entry:

   • Ensure the NAT ACL includes traffic from the ISP to internal networks.

2. Return Traffic Blocked:

   • Add a reflexive ACL or an explicit ACL to permit return traffic.

3. Incorrect NAT Outside Interface:

   • Confirm the ip nat outside interface matches the one facing the ISP.

4. No Static NAT:

   • If specific services (like pings) need to be reachable, configure static NAT:
     bash

     ip nat inside source static <Internal_IP> <Public_IP>

[Joseph W. Doherty]

Ignoring other considerations, BTW, if true dynamic NAT is being used, an external host could only reach an internal host if there's an active NAT entry for the two hosts.  True static NAT would work provided a static NAT entry is in place for the two hosts.

In the forgoing, I used the adjective "true", i.e. just NAT not NAT/PAT.

If PAT, or complex variant, is in play, often the inside to outside also has some additional state information for expected reverse flow traffic.  If such criteria isn't met, outside won't be able to contact an inside host.