hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Router R1 is configured with ip nat inside source…
Router R1 is configured with ip nat inside source list 100 interface Loopback0 overload. Internal hosts at 192.168.1.0/24 can access the internet, but external hosts cannot initiate connections to an internal server at 10.1.1.10 that is also behind NAT. The server is supposed to be reachable via static NAT. Configuration: ip nat inside source static tcp 10.1.1.10 80 interface Loopback0 80. Router R1 shows: show ip nat translations: Pro Inside global Inside local Outside local Outside global tcp 10.1.1.10:80 10.1.1.10:80 --- ---. External users get connection timeouts. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between using 'interface' for dynamic PAT and a specific IP for static NAT, trapping candidates who think 'interface' can serve both purposes without understanding that static NAT requires a fixed global address for inbound reachability.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The static NAT should use a specific global IP address instead of the interface; configure ip nat inside source static tcp 10.1.1.10 80 203.0.113.10 80.
The static NAT configuration uses 'interface Loopback0' as the global address, which means it relies on the interface's dynamically assigned IP address. However, the overload rule (PAT) is also using the same interface, and the static NAT entry shown in 'show ip nat translations' displays the inside local address as the inside global address (10.1.1.10:80), indicating that the static translation is not being applied correctly. The root cause is that static NAT requires a specific global IP address (e.g., 203.0.113.10) to map the internal server, not the interface keyword, because the interface keyword is used for PAT and does not create a fixed one-to-one mapping for inbound connections.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The static NAT should use a specific global IP address instead of the interface; configure ip nat inside source static tcp 10.1.1.10 80 203.0.113.10 80.
Why this is correct
Using the interface IP for static NAT is not supported; a specific global IP must be defined.
- ✗
The access-list 100 is blocking inbound traffic.
Why it's wrong here
ACL 100 is for dynamic NAT, not static NAT, and inbound traffic is not matched by it.
- ✗
The Loopback0 interface is not in the routing table.
Why it's wrong here
Loopback0 is a virtual interface and is always up, but it is not the correct interface for NAT.
- ✗
The static NAT entry is missing the 'add-route' option.
Why it's wrong here
The 'add-route' option is for route maps, not required for static NAT.
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