- A
The Null0 interface is administratively shut down.
Why wrong: Null0 is a virtual interface that is always in the up/up state and never requires a shutdown command. A static route pointing to Null0 is valid regardless of interface state.
- B
The address 0.0.0.0/0 is an invalid destination for a static route because it is not a real network.
Why wrong: The 0.0.0.0 0.0.0.0 prefix is the default route and is perfectly valid in static routing. Discard routes are commonly configured with this prefix to drop traffic when no more specific route exists.
- C
The static discard route must include a next-hop IP address instead of specifying Null0 as the interface.
Why wrong: A static route to Null0 is a standard method to create a discard route by specifying the outgoing interface as Null0 without any next-hop address. This syntax is fully supported.
- D
The ISP static default route has an administrative distance of 1, which is lower than the administrative distance of 254 set for the Null0 route.
When a router has multiple routes for the same 0.0.0.0/0 prefix, it selects the one with the lowest administrative distance. The ISP route with AD 1 wins, so traffic never matches the AD 254 Null0 discard route.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
After configuring a static discard route with the command 'ip route 0.0.0.0 0.0.0.0 Null0 254' on R1, a network engineer finds that traffic to destinations that are not in the routing table is still being forwarded out to the ISP instead of being dropped. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 ISP static default route has an administrative distance of 1, which is lower than the administrative distance of 254 set for the Null0 route.
The static default route via the ISP has an administrative distance of 1 (either explicitly configured or inherited as a static route), which is lower than the Null0 route's AD of 254. Because the router always prefers the route with the lowest administrative distance for the same destination, the ISP route is installed in the forwarding table, and the discard route is never used. For the discard route to be selected, its AD would need to be less than or equal to the ISP route's AD, or the ISP route must be removed.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 Null0 interface is administratively shut down.
Why it's wrong here
Null0 is a virtual interface that is always in the up/up state and never requires a shutdown command. A static route pointing to Null0 is valid regardless of interface state.
- ✗
The address 0.0.0.0/0 is an invalid destination for a static route because it is not a real network.
Why it's wrong here
The 0.0.0.0 0.0.0.0 prefix is the default route and is perfectly valid in static routing. Discard routes are commonly configured with this prefix to drop traffic when no more specific route exists.
- ✗
The static discard route must include a next-hop IP address instead of specifying Null0 as the interface.
Why it's wrong here
A static route to Null0 is a standard method to create a discard route by specifying the outgoing interface as Null0 without any next-hop address. This syntax is fully supported.
- ✓
The ISP static default route has an administrative distance of 1, which is lower than the administrative distance of 254 set for the Null0 route.
Why this is correct
When a router has multiple routes for the same 0.0.0.0/0 prefix, it selects the one with the lowest administrative distance. The ISP route with AD 1 wins, so traffic never matches the AD 254 Null0 discard route.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The ISP static default route has an administrative distance of 1, which is lower than the administrative distance of 254 set for the Null0 route.Correct answer▾
Why this is correct
When a router has multiple routes for the same 0.0.0.0/0 prefix, it selects the one with the lowest administrative distance. The ISP route with AD 1 wins, so traffic never matches the AD 254 Null0 discard route.
✗The Null0 interface is administratively shut down.Wrong answer — click to see why▾
Why this is wrong here
Candidates often assume that a route pointing to an interface requires that interface to be up, but Null0 is always available.
✗The address 0.0.0.0/0 is an invalid destination for a static route because it is not a real network.Wrong answer — click to see why▾
Why this is wrong here
A misconception that the default route is special and cannot be used with Null0, but any valid IP prefix can be directed to Null0 to create a discard route.
✗The static discard route must include a next-hop IP address instead of specifying Null0 as the interface.Wrong answer — click to see why▾
Why this is wrong here
Confusing Null0 routes with Ethernet interface routes, which require a next hop, leads to the incorrect belief that a next hop is mandatory for all static routes.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
Null0 is a virtual interface that is always in the up/up state and never requires a shutdown command. A static route pointing to Null0 is valid regardless of interface state.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ISP static default route has an administrative distance of 1, which is lower than the administrative distance of 254 set for the Null0 route. — The static default route via the ISP has an administrative distance of 1 (either explicitly configured or inherited as a static route), which is lower than the Null0 route's AD of 254. Because the router always prefers the route with the lowest administrative distance for the same destination, the ISP route is installed in the forwarding table, and the discard route is never used. For the discard route to be selected, its AD would need to be less than or equal to the ISP route's AD, or the ISP route must be removed.
What should I do if I get this 200-301 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 14, 2026
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.
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