The answer is 10.5.5.5/32, as it is the only prefix not reachable via MPLS forwarding based on the LDP label bindings shown in the exhibit. MPLS forwarding relies on label-switched paths built from LDP label bindings stored in the label information base (LIB); without a binding for 10.5.5.5/32, the router cannot impose a label and must fall back to standard IP routing. On the Cisco SPCOR 350-501 exam, this scenario tests your ability to read an LDP LIB output and identify which prefixes have been assigned labels, a common trap being that all loopbacks in the topology are assumed to have labels when only those learned via LDP do. A key memory tip is to remember that LDP only assigns labels to prefixes in its routing table that are directly connected or learned via an IGP—if a prefix is missing from the LIB, it simply has no MPLS label and cannot be label-switched.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
R1# show mpls ldp bindings
label bindings: 5
label binding: 10.1.1.1/32, label 16
label binding: 10.2.2.2/32, label 17
label binding: 10.3.3.3/32, label 18
label binding: 10.4.4.4/32, label 19
label binding: 10.5.5.5/32, label 20
R1# show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
16 Pop Label 10.1.1.1/32 0 Gi0/0 192.168.1.2
17 18 10.2.2.2/32 0 Gi0/0 192.168.1.2
18 19 10.3.3.3/32 0 Gi0/0 192.168.1.2
19 20 10.4.4.4/32 0 Gi0/0 192.168.1.2
20 Untagged 10.5.5.5/32 0 Gi0/0 192.168.1.2
Based on the exhibit, which prefix is NOT reachable via MPLS forwarding?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "NOT"
Why it matters: Negative qualifier — you are looking for the one option that does NOT apply. Most options will be true; only one is false for this scenario.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
10.5.5.5/32
Option D (10.5.5.5/32) is correct because the exhibit shows that the LDP label bindings are only present for prefixes 10.1.1.1/32, 10.2.2.2/32, and 10.3.3.3/32. The prefix 10.5.5.5/32 is not in the LDP label information base (LIB), meaning no MPLS label has been assigned to it, so it cannot be forwarded via MPLS and must be forwarded using standard IP routing instead.
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.
✗
10.1.1.1/32
Why it's wrong here
It has Pop Label, which is valid for MPLS forwarding.
✗
10.3.3.3/32
Why it's wrong here
It has an outgoing label of 19, so it is forwarded via MPLS.
✗
10.2.2.2/32
Why it's wrong here
It has an outgoing label of 18, so it is forwarded via MPLS.
✓
10.5.5.5/32
Why this is correct
The outgoing label is Untagged, so MPLS forwarding is not applied, likely causing packet drop.
Clue confirmation
The clue word "NOT" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between prefixes that have LDP label bindings versus those that are simply present in the routing table, trapping candidates who assume all IGP routes are automatically MPLS-switched without verifying the label bindings.
Detailed technical explanation
How to think about this question
LDP (Label Distribution Protocol) assigns labels to IGP-learned prefixes, and the absence of a label binding for 10.5.5.5/32 indicates that either the route is not in the routing table, LDP is not enabled on the interface toward that prefix, or the neighbor has not advertised a label. In MPLS networks, only prefixes with an assigned label can be switched via label swapping; unlabeled packets are forwarded using the IP routing table, which may lead to suboptimal paths or loss of MPLS benefits like traffic engineering.
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 350-501 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.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 10.5.5.5/32 — Option D (10.5.5.5/32) is correct because the exhibit shows that the LDP label bindings are only present for prefixes 10.1.1.1/32, 10.2.2.2/32, and 10.3.3.3/32. The prefix 10.5.5.5/32 is not in the LDP label information base (LIB), meaning no MPLS label has been assigned to it, so it cannot be forwarded via MPLS and must be forwarded using standard IP routing instead.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "NOT". Negative qualifier — you are looking for the one option that does NOT apply. Most options will be true; only one is false for this scenario.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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