CCNA IP Routing Practice Question
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?
⚠ Common exam trap
Cisco often tests the concept that administrative distance determines route preference, and the trap here is that candidates assume a static route to Null0 will always be used for discarding traffic, forgetting that a lower-AD route (like the ISP default) will take precedence.
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 discard route with an administrative distance of 254 is less preferred than the ISP default route with an administrative distance of 1. Since the ISP route has a lower AD, it remains in the routing table and is used for forwarding traffic to unknown destinations, overriding the discard route. The Null0 route only becomes active if the ISP route is removed or fails, as the router always selects the route with the lowest administrative distance.
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 on Cisco routers that is always in an up/up state and cannot be administratively shut down. The interface is a built-in discard destination, so it never needs a shutdown or no shutdown command, and it always accepts traffic for immediate dropping. Therefore, the Null0 interface's operational state is not the reason the discard route is failing to drop traffic.
- ✗
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 (often written as 0.0.0.0/0) is the default route and is a perfectly valid destination for a static route. In fact, a static route to Null0 with this prefix is a standard technique for creating a discard route that drops all traffic not matching a more specific route. The validity of the prefix is not in question; the problem lies in the route preference, not the destination address.
- ✗
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 does not require a next-hop IP address. The syntax 'ip route 0.0.0.0 0.0.0.0 Null0' is a legitimate way to create a discard route, and Cisco IOS explicitly allows specifying the Null0 interface as the outgoing interface with no next-hop. The issue is not the lack of a next-hop but rather the administrative distance, which determines which of the two default routes is actually installed in the routing table.
- ✓
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.
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?”
Go deeper
Related to this question
Learn chapter
Interpreting HSRP and VRRP Operational Status
Key term
ISP
An ISP is a company that provides individuals and organizations access to the Internet, along with related services like email and web hosting.
Key term
Administrative distance
Administrative distance is a number that a router uses to decide which routing protocol's route to trust when it learns about the same destination from multiple different routing protocols.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.