Quick Answer
ECMP with a hash algorithm based on Layer 3 and Layer 4 headers is the correct choice because it provides per-flow load balancing across multiple equal-cost paths in an MPLS network while preserving packet order. The hash is computed over source and destination IP addresses along with TCP or UDP port numbers, which ensures that all packets sharing the same five-tuple are consistently mapped to the same next-hop, preventing reordering while still distributing distinct flows across the available links. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how MPLS leverages ECMP to avoid out-of-order delivery, a common trap where candidates mistakenly choose round-robin or per-packet load balancing, which would break packet order. Remember the key distinction: per-flow hashing uses Layer 3 and Layer 4 entropy, not just Layer 2 or a simple round-robin. A helpful memory tip is “Hash the five-tuple to keep the flow stable.”
This 350-501 practice question tests your understanding of networking. 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.
Which technology should be used to provide per-flow load balancing across multiple equal-cost paths in an MPLS network while preserving packet order?
ECMP with a hash algorithm based on layer 3 and layer 4 headers
ECMP hashing preserves flow ordering.
LAG with default hashing
Why wrong: LAG is for link aggregation, not per-flow across ECMP.
Policy-based routing with next-hop per destination
Why wrong: PBR does not provide per-flow load balancing.
Per-packet round-robin load balancing
Why wrong: Per-packet may reorder packets.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ECMP with a hash algorithm based on layer 3 and layer 4 headers
ECMP with a hash algorithm based on Layer 3 and Layer 4 headers is the correct choice because it provides per-flow load balancing by computing a hash over source/destination IP addresses and TCP/UDP port numbers, ensuring that all packets belonging to the same flow are forwarded over the same equal-cost path. This preserves packet order while distributing different flows across multiple paths in an MPLS network.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
ECMP with a hash algorithm based on layer 3 and layer 4 headers
Why this is correct
ECMP hashing preserves flow ordering.
Related concept
Read the scenario before looking for a memorised answer.
LAG with default hashing
Why it's wrong here
LAG is for link aggregation, not per-flow across ECMP.
Policy-based routing with next-hop per destination
Why it's wrong here
PBR does not provide per-flow load balancing.
Per-packet round-robin load balancing
Why it's wrong here
Per-packet may reorder packets.
Common exam traps
Cisco often tests the distinction between per-flow and per-packet load balancing, and the trap here is that candidates confuse ECMP with per-packet round-robin, assuming both preserve order, but only per-flow hashing guarantees packet ordering within a flow.
Detailed technical explanation
In MPLS networks, ECMP uses a hash over the IP header (and optionally MPLS labels) to select a path; Cisco IOS uses the 'mpls ip' command with 'load-balance per-packet' or 'per-flow' algorithms, but per-flow hashing is the default for ECMP. The hash is typically computed using a CRC or XOR of the 5-tuple (source IP, destination IP, protocol, source port, destination port), and the result is used to index into a next-hop table, ensuring flow affinity even when MPLS labels are swapped. A real-world scenario is a service provider core where multiple LSPs exist between PEs; per-flow hashing prevents TCP retransmissions caused by out-of-order delivery.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: ECMP with a hash algorithm based on layer 3 and layer 4 headers — ECMP with a hash algorithm based on Layer 3 and Layer 4 headers is the correct choice because it provides per-flow load balancing by computing a hash over source/destination IP addresses and TCP/UDP port numbers, ensuring that all packets belonging to the same flow are forwarded over the same equal-cost path. This preserves packet order while distributing different flows across multiple paths in an MPLS network.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This 350-501 practice question is part of Courseiva's free Cisco 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 350-501 exam.
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