JN0-106 Routing Fundamentals Practice Question
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?
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.
The correct answer is B: the router is likely not receiving the ISP-A routes for those specific prefixes, perhaps because ISP-A's BGP session is missing or the prefix is not advertised. Local-preference is evaluated before MED and IGP metric in the BGP best-path selection process, so if a route from ISP-A with local-preference 200 existed for those destinations, it would be preferred over the ISP-B route with local-preference 100. If ISP-A simply isn't advertising those prefixes (or the session is down), no ISP-A route exists to compare, so the ISP-B route becomes active. Option A is false because local-preference is a well-known discretionary attribute that is commonly set on eBGP-learned routes via inbound policy. Option C is wrong because IGP metric is only considered after local-preference and MED, so it cannot override a higher local-preference. Option D is wrong because MED is compared after local-preference, so a higher MED from ISP-A would not cause the lower-local-preference ISP-B route to win.
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 a well-known discretionary BGP attribute that applies to routes learned from any neighbor, including eBGP, not just iBGP. In practice, local-preference is most often set via import policy on eBGP peerings to influence outbound traffic, and it is then carried over iBGP inside the AS. It is non-transitive, meaning it is never advertised to eBGP peers, but it absolutely works on eBGP-learned routes. Therefore the statement is false.
- ✓
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
BGP best-path selection can only evaluate routes that are actually present in the BGP table. If the prefixes are not being received from ISP-A—because the eBGP session is down, the neighbor is not advertising them, or inbound filtering is discarding them—the router has only the ISP-B routes to consider, so the local-preference setting for ISP-A is irrelevant. The route from ISP-B then wins by default, exactly matching the observed symptoms.
- ✗
The IGP metric to the next-hop from ISP-B is lower, causing the route to be preferred.
Why it's wrong here
IGP metric is a tie-breaking criterion that is evaluated only after many higher-priority BGP attributes, including local-preference, AS-path length, origin, and MED. Even if the IGP metric to ISP-B's next-hop is lower, that comparison is never reached when the local-preference of ISP-A's route is higher. BGP does not use IGP metric to compare routes from different autonomous systems; it only breaks ties between multiple otherwise-equal BGP routes. Therefore a lower IGP metric cannot be the reason ISP-B is preferred.
- ✗
The MED value from ISP-A is higher than from ISP-B, overriding the local-preference.
Why it's wrong here
MED (Multi-Exit Discriminator) is compared after local-preference and AS-path length, not before, so it cannot override a difference in local-preference. If ISP-A's route has a higher local-preference, BGP selects it regardless of whether ISP-A's MED is higher or lower than ISP-B's MED. For MED to matter, the candidate routes must first be equal in weight, local-preference, and AS-path length. Thus a higher MED from ISP-A does not explain why ISP-B is chosen.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 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 JN0-106 exam.