- A
Junos uses per-flow load balancing based on source and destination IP and port.
Junos defaults to per-flow load balancing using a hash of source and destination IP addresses and ports.
- B
Junos uses the route with the lowest next-hop IP.
Why wrong: The next-hop IP is a tie-breaker after preference and metric, but the router still installs all equal-cost paths.
- C
Junos installs only one route and ignores the others.
Why wrong: Junos installs multiple next-hops for ECMP, not ignoring others.
- D
Junos uses per-packet load balancing by default.
Why wrong: Per-packet load balancing is not the default; Junos uses per-flow.
Question 104 of 514
Junos Default ECMP Load Balancing: Per-Flow Based on IP and Port
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?
Quick Answer
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.
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
✓
Junos uses per-flow load balancing based on source and destination IP and port.
When multiple equal-cost paths exist to the same destination, Junos uses per-flow load balancing by default. It hashes the source IP, destination IP, source port, and destination port (for TCP/UDP) to consistently assign each flow to a single next-hop, ensuring packet order is preserved per flow.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
Read the scenario before looking for a memorised answer.
- ✗
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: answer the scenario, not the keyword
The trap here is that Cisco IOS defaults to per-packet load balancing over equal-cost paths, while Junos defaults to per-flow load balancing, leading candidates familiar with Cisco to incorrectly select per-packet as the default.
Detailed technical explanation
How to think about this question
The hash used for per-flow load balancing in Junos is based on a 5-tuple (source IP, destination IP, protocol, source port, destination port) for TCP/UDP, or a 3-tuple (source IP, destination IP, protocol) for other protocols. This hash is computed in hardware on the line cards, allowing high-speed forwarding decisions without per-packet CPU intervention. In a real-world scenario, if a BGP speaker advertises two equal-cost paths, Junos will load balance flows across both links, but a single flow (e.g., a large file transfer) will always use the same path, avoiding TCP reordering issues.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Routing Fundamentals — study guide chapter
Learn the concepts, then practise the questions
- →
Routing Fundamentals practice questions
Targeted practice on this topic area only
- →
All JNCIA-JUNOS questions
514 questions across all exam domains
- →
Juniper Networks Certified Associate Junos JNCIA-Junos study guide
Full concept coverage aligned to exam objectives
- →
JNCIA-JUNOS practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related JNCIA-JUNOS practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
User Interfaces practice questions
Practise JNCIA-JUNOS questions linked to User Interfaces.
Junos Configuration Basics practice questions
Practise JNCIA-JUNOS questions linked to Junos Configuration Basics.
Operational Monitoring and Maintenance practice questions
Practise JNCIA-JUNOS questions linked to Operational Monitoring and Maintenance.
Routing Fundamentals practice questions
Practise JNCIA-JUNOS questions linked to Routing Fundamentals.
Networking Fundamentals practice questions
Practise JNCIA-JUNOS questions linked to Networking Fundamentals.
Junos OS Fundamentals practice questions
Practise JNCIA-JUNOS questions linked to Junos OS Fundamentals.
JNCIA-JUNOS fundamentals practice questions
Practise JNCIA-JUNOS questions linked to JNCIA-JUNOS fundamentals.
JNCIA-JUNOS scenario practice questions
Practise JNCIA-JUNOS questions linked to JNCIA-JUNOS scenario.
JNCIA-JUNOS troubleshooting practice questions
Practise JNCIA-JUNOS questions linked to JNCIA-JUNOS troubleshooting.
Practice this exam
Start a free JNCIA-JUNOS practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Routing Fundamentals — This question tests Routing Fundamentals — Read the scenario before looking for a memorised answer..
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. — When multiple equal-cost paths exist to the same destination, Junos uses per-flow load balancing by default. It hashes the source IP, destination IP, source port, and destination port (for TCP/UDP) to consistently assign each flow to a single next-hop, ensuring packet order is preserved per flow.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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 →
Same concept, more angles
1 more ways this is tested on JNCIA-JUNOS
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: Option B is correct because per-flow load balancing in Junos requires the 'multipath' statement under 'routing-options' to enable ECMP (Equal-Cost Multipath) for OSPF. Without this, Junos will only install a single best path even if multiple equal-cost routes exist. Once 'multipath' is configured, the system automatically uses per-flow load balancing based on the default hash key (source/destination IP and optionally Layer 4 ports).
Keep practising
More JNCIA-JUNOS practice questions
- A junior engineer needs to collect a snapshot of the current system state, including routing tables, interfaces, and con…
- A network engineer is troubleshooting a connectivity issue and wants to see the active routes in the routing table. Whic…
- You are the network engineer for a mid-sized enterprise with a Juniper MX router running Junos. The router has two uplin…
- Which TWO attributes are used by Junos to select the active route among multiple routes to the same destination?
- What is the default preference of a direct route in Junos?
- A router has two routes to the same destination: one with preference 10 and metric 5, and another with preference 15 and…
Last reviewed: Jul 4, 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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.