350-501 MPLS and Segment Routing Practice Question
Which TWO statements about MPLS label switching are correct? (Choose two.)
⚠ Common exam trap
Cisco often tests the distinction between which router performs label popping in PHP (penultimate hop vs. egress) and which router swaps labels (transit LSR vs. egress LSR), leading candidates to confuse the roles of the penultimate and egress routers.
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
✓
The transit LSR performs label swapping.
A transit Label Switch Router (LSR) in an MPLS network performs label swapping: it receives a labeled packet, replaces the incoming label with an outgoing label from its LFIB (Label Forwarding Information Base), and forwards the packet toward the egress LSR. This is the fundamental operation of an LSR in the core of an MPLS domain, as defined in RFC 3031.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The transit LSR performs label swapping.
Why this is correct
Correct: Transit routers swap the incoming label with an outgoing label.
- ✗
The CE receives a frame with an MPLS label.
Why it's wrong here
CE receives an IP packet without label (unless configured for MPLS).
- ✓
The ingress LSR imposes a label on the packet.
Why this is correct
Correct: Ingress does label imposition (push).
- ✗
PHP (Penultimate Hop Popping) causes the egress router to pop the label.
Why it's wrong here
PHP causes the penultimate hop to pop the label, not the egress.
- ✗
The egress LSR performs label swapping before forwarding.
Why it's wrong here
Egress pops the label, not swap.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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