350-501 MPLS and Segment Routing Practice Question
Which two statements about MPLS label operations are true? (Choose two.)
⚠ Common exam trap
Cisco often tests the misconception that push and pop operations are limited to one label at a time, but push can impose multiple labels (e.g., for label stacking), and pop with implicit-null is a specific mechanism to optimize egress processing.
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
✓
Push operation adds a label to the packet.
The MPLS push operation adds a new label onto the top of the label stack, encapsulating the packet for forwarding through an LSP. Option E is correct because the pop operation removes the top label, and implicit-null (label value 3) triggers the penultimate hop to pop the label before forwarding the packet to the egress LSR, reducing the egress router's processing load.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Push operation can only add one label at a time.
Why it's wrong here
Push can add a label stack of multiple labels.
- ✗
Pop operation can remove multiple labels at once.
Why it's wrong here
Pop removes exactly one label per operation.
- ✓
Push operation adds a label to the packet.
Why this is correct
Push adds an MPLS header.
- ✗
Swap operation removes and replaces two labels.
Why it's wrong here
Swap replaces the top label with a new one.
- ✓
Pop operation removes the top label and may be triggered by implicit-null.
Why this is correct
Pop removes the label; implicit-null causes pop at penultimate hop.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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