- A
The local-preference is not applied to routes that are learned via eBGP; it only works for iBGP.
Why wrong: Local-preference is applied to both eBGP and iBGP routes.
- B
The router is not receiving the ISP-A routes for those specific prefixes; perhaps ISP-A's BGP session is missing or the prefix is not advertised.
If only ISP-B has the route, it will be active regardless of local-preference.
- C
The IGP metric to the next-hop from ISP-B is lower, causing the route to be preferred.
Why wrong: IGP metric is considered later in BGP selection, after local-preference.
- D
The MED value from ISP-A is higher than from ISP-B, overriding the local-preference.
Why wrong: MED is compared after local-preference and AS-path; it cannot override local-preference if local-preference differs.
Quick Answer
The answer is that the router is not receiving the ISP-A routes for those specific prefixes, likely because ISP-A's BGP session is down or not advertising those networks. This is correct because BGP local preference is the first tiebreaker in the best path selection algorithm on Junos; a route with local-preference 200 will always be chosen over one with 100 if both are present. If the ISP-B route is active despite its lower local-preference, it means the ISP-A route simply does not exist in the routing table for those destinations, forcing the router to use the backup. On the JNCIA-Junos exam, this scenario tests your understanding that local preference is evaluated before attributes like AS-path length or MED, and a common trap is assuming a higher local preference guarantees a route is active—it only applies if the route is actually received. Remember the memory tip: "Local preference first, but no route means no choice."
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. 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.
Your company recently acquired a small office that uses a Juniper MX router to connect to two ISPs for redundancy. The router has two uplinks: xe-0/0/0 to ISP-A (next-hop 10.0.0.1) and xe-0/0/1 to ISP-B (next-hop 10.0.1.1). The router receives a full BGP table from both ISPs. You want to prefer ISP-A for most traffic, but use ISP-B as a backup. You have configured BGP with local-preference 200 on routes from ISP-A and local-preference 100 on routes from ISP-B. After committing, you check the routing table and see that for some destinations, the route from ISP-B is active despite having lower local-preference. What is the most likely reason?
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 router is not receiving the ISP-A routes for those specific prefixes; perhaps ISP-A's BGP session is missing or the prefix is not advertised.
Option A is correct because BGP best path selection considers local-preference first, but if multiple routes have the same local-preference, other attributes like AS-path length are considered. Since both routes have different local-preference (200 vs 100), the one with 200 should be preferred. If the ISP-B route is active for some destinations, it indicates that the ISP-A route might not be received for those prefixes (e.g., partial BGP table). Option B is incorrect; MED is compared only if the routes are from the same AS. Option C is incorrect; IGP metric is not compared in BGP best path selection until later steps, and local-preference is first. Option D is incorrect; the router does not ignore local-preference.
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 local-preference is not applied to routes that are learned via eBGP; it only works for iBGP.
Why it's wrong here
Local-preference is applied to both eBGP and iBGP routes.
- ✓
The router is not receiving the ISP-A routes for those specific prefixes; perhaps ISP-A's BGP session is missing or the prefix is not advertised.
Why this is correct
If only ISP-B has the route, it will be active regardless of local-preference.
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.
- ✗
The IGP metric to the next-hop from ISP-B is lower, causing the route to be preferred.
- ✗
The MED value from ISP-A is higher than from ISP-B, overriding the local-preference.
Why it's wrong here
MED is compared after local-preference and AS-path; it cannot override local-preference if local-preference differs.
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.
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 JNCIA-JUNOS NAT questions on configuration and troubleshooting.
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Routing Fundamentals — study guide chapter
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Routing Fundamentals — This question tests Routing Fundamentals — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The router is not receiving the ISP-A routes for those specific prefixes; perhaps ISP-A's BGP session is missing or the prefix is not advertised. — Option A is correct because BGP best path selection considers local-preference first, but if multiple routes have the same local-preference, other attributes like AS-path length are considered. Since both routes have different local-preference (200 vs 100), the one with 200 should be preferred. If the ISP-B route is active for some destinations, it indicates that the ISP-A route might not be received for those prefixes (e.g., partial BGP table). Option B is incorrect; MED is compared only if the routes are from the same AS. Option C is incorrect; IGP metric is not compared in BGP best path selection until later steps, and local-preference is first. Option D is incorrect; the router does not ignore local-preference.
What should I do if I get this JNCIA-JUNOS 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 JNCIA-JUNOS 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 24, 2026
This JNCIA-JUNOS practice question is part of Courseiva's free Juniper Networks 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 JNCIA-JUNOS exam.
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