The answer is that the BGP routes are from different autonomous systems. By default, Junos BGP does not install multiple routes from different ASes into the routing table for load balancing, even when all other attributes like local preference, AS path length, and metric are identical. The router selects only the best path, and since both routes are from distinct ISPs with different AS numbers, the second route remains inactive unless the 'multipath multiple-as' command is explicitly configured under the BGP group. This question tests your understanding of BGP multipath behavior on the JNCIA-Junos exam, a common trap where candidates assume equal attributes guarantee load balancing, forgetting the default single-AS restriction. A useful memory tip: think of "multiple AS" as needing the "multiple-as" knob—without it, Junos treats different AS paths as separate best-path candidates, not equal-cost multipath entries.
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.
A service provider operates a Juniper MX router that receives a default route from two different ISPs via EBGP. Both ISPs advertise 0.0.0.0/0 with the same local preference (100), same AS path length, and same metric. The network administrator wants to load balance outgoing traffic across both ISPs for redundancy and bandwidth utilization. After applying the configuration, the administrator checks the routing table and sees that only one default route is active, the one from ISP-A. The other route from ISP-B is present but not active. The administrator verifies that both routes have the same preference (170) and same BGP attributes. The ISP links are different: ISP-A is connected via interface ge-0/0/0.0 with next-hop 10.1.1.2, and ISP-B via interface ge-0/0/1.0 with next-hop 10.2.2.2. What is the most likely reason that both routes are not active and load-balanced?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The BGP routes are from different autonomous systems.
The correct answer is C. By default, Junos BGP does not load balance routes from different autonomous systems. To enable multipath across different ASes, the 'multipath multiple-as' command must be configured under BGP. Option A is irrelevant. Option B is not a cause because different next-hops can be load-balanced under multipath. Option D is the solution, not the cause. Therefore, the most likely cause is that the routes are from different ASes.
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 router has 'no-policy-statistics' enabled on the BGP sessions.
Why it's wrong here
This setting does not affect route selection or load balancing.
✗
The BGP routes have different next-hop addresses.
Why it's wrong here
Different next-hops are allowed in BGP multipath; this is not a blocking condition.
✓
The BGP routes are from different autonomous systems.
Why this is correct
Junos requires the 'multipath multiple-as' configuration to load balance routes from different ASes; otherwise, only the best path is installed.
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 router is configured with 'multipath multiple-as' under BGP.
Why it's wrong here
This is a solution, not a cause. If it were configured, load balancing would work.
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.
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 BGP routes are from different autonomous systems. — The correct answer is C. By default, Junos BGP does not load balance routes from different autonomous systems. To enable multipath across different ASes, the 'multipath multiple-as' command must be configured under BGP. Option A is irrelevant. Option B is not a cause because different next-hops can be load-balanced under multipath. Option D is the solution, not the cause. Therefore, the most likely cause is that the routes are from different ASes.
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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