The answer is Downstream Unsolicited with Liberal Label Retention. This is the correct combination because LDP offers two label distribution modes—Downstream Unsolicited (DU) and Downstream-on-Demand (DoD)—and each can pair with either liberal or conservative label retention, or with ordered or independent control. DU with liberal label retention is the default and most common mode in Cisco MPLS networks, where each LSR proactively advertises labels for all known FECs and retains all received labels, even if not immediately needed. On the Cisco SPCOR 350-501 exam, this question tests your understanding of valid LDP label distribution modes versus invalid mixes; a common trap is confusing control modes (ordered/independent) with retention modes (liberal/conservative) or assuming DoD can pair with liberal retention. Remember the mnemonic: “DU is default, DoD is demand; liberal keeps all, conservative drops.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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.
An engineer is configuring LDP in an MPLS network. Which THREE are valid label distribution modes for LDP?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Downstream on Demand with Independent Label Distribution Control
LDP has two label distribution modes: DoD (Downstream-on-Demand) and DU (Downstream Unsolicited). DU can be either liberal or conservative label retention. Control mode can be ordered or independent. The combination of distribution and retention/control defines the mode. Classic modes: DU liberal (default), DU conservative, DoD ordered, DoD independent. Options C, D, E are not valid combinations.
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.
✓
Downstream on Demand with Independent Label Distribution Control
Why this is correct
This is a valid combination.
Related concept
Static NAT maps one inside address to one outside address.
✓
Downstream Unsolicited with Conservative Label Retention
Why this is correct
This is valid, though less common.
Related concept
Static NAT maps one inside address to one outside address.
✓
Downstream Unsolicited with Liberal Label Retention
Why this is correct
This is the default mode for LDP.
Related concept
Static NAT maps one inside address to one outside address.
✗
Downstream Unsolicited with Ordered Label Distribution Control
Why it's wrong here
Ordered control is associated with DoD, not DU.
✗
Downstream on Demand with Liberal Label Retention
Why it's wrong here
DoD mode uses conservative label retention inherently.
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 350-501 NAT questions on configuration and troubleshooting.
Architecture — This question tests Architecture — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Downstream on Demand with Independent Label Distribution Control — LDP has two label distribution modes: DoD (Downstream-on-Demand) and DU (Downstream Unsolicited). DU can be either liberal or conservative label retention. Control mode can be ordered or independent. The combination of distribution and retention/control defines the mode. Classic modes: DU liberal (default), DU conservative, DoD ordered, DoD independent. Options C, D, E are not valid combinations.
What should I do if I get this 350-501 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 350-501 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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