The answer is per-flow load balancing based on source and destination IP and port. Junos uses this default behavior for ECMP load balancing to ensure that all packets belonging to the same traffic flow—identified by the unique combination of source IP, destination IP, source port, and destination port—are sent over the same next-hop path, which prevents packet reordering and maintains session integrity. On the JNCIA-Junos exam, this concept tests your understanding of how Junos handles multiple equal-cost routes, and a common trap is confusing per-flow with per-packet load balancing, which is not the default. Remember that Junos prioritizes flow affinity over raw distribution, so it will not spray individual packets across links. A helpful memory tip is to think of the four-tuple (IPs and ports) as the “flow fingerprint” that Junos hashes to pick a single path, keeping each conversation intact.
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 router has multiple equal-cost paths to the same destination. Which statement describes how Junos load balances traffic across these paths?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Junos uses per-flow load balancing based on source and destination IP and port.
Junos uses per-flow load balancing by default based on source/destination IP and port. Option A (per-packet) is not default. Option B is false. Option D is not the primary method for ECMP.
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.
✓
Junos uses per-flow load balancing based on source and destination IP and port.
Why this is correct
Junos defaults to per-flow load balancing using a hash of source and destination IP addresses and ports.
Related concept
Static NAT maps one inside address to one outside address.
✗
Junos uses the route with the lowest next-hop IP.
Why it's wrong here
The next-hop IP is a tie-breaker after preference and metric, but the router still installs all equal-cost paths.
✗
Junos installs only one route and ignores the others.
Why it's wrong here
Junos installs multiple next-hops for ECMP, not ignoring others.
✗
Junos uses per-packet load balancing by default.
Why it's wrong here
Per-packet load balancing is not the default; Junos uses per-flow.
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: Junos uses per-flow load balancing based on source and destination IP and port. — Junos uses per-flow load balancing by default based on source/destination IP and port. Option A (per-packet) is not default. Option B is false. Option D is not the primary method for ECMP.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network has two equal-cost OSPF paths to 192.168.1.0/24. The engineer wants to enable per-flow load balancing. Which configuration element is required?
medium
A.Configure per-packet load balancing under the interface
✓ B.Add 'multipath' under 'routing-options' for OSPF
C.Define a load-balancing hash-key including Layer 4 information
D.Set a higher preference on one path to force ECMP
Why B: ECMP requires enabling the 'multipath' statement under routing-options for the protocol. Option A is correct. Option B is not a typical configuration; per-packet is not recommended. Option C is incorrect because setting a higher preference would break ECMP. Option D is incorrect because the hash-key is optional and not required.
Last reviewed: Jun 24, 2026
Question Discussion
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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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