350-501 Architecture Practice Question
Which TWO statements about MPLS label operations in a service provider core are correct? (Choose two.)
⚠ Common exam trap
Cisco often tests the misconception that the penultimate hop performs a label swap, when in fact it performs a pop (PHP) by default, and that the egress router always receives two labels, which is only true in specific scenarios like MPLS VPNs with a transport label and a VPN label.
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 ingress router pushes a label stack onto the IP packet
The ingress router (LER) in an MPLS network performs a push operation, adding a label stack (typically one or more labels) onto the incoming IP packet. This label stack is used to direct the packet along a Label Switched Path (LSP) through the core, enabling MPLS forwarding based on labels rather than IP routing.
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 ingress router pushes a label stack onto the IP packet
Why this is correct
Ingress pushes the label stack to encapsulate the packet.
- ✗
The egress router receives an MPLS packet with two labels by default
Why it's wrong here
After PHP, the egress receives a single label or IP packet.
- ✗
The penultimate hop performs label swap for the top label
Why it's wrong here
Penultimate hop performs label pop (PHP), not swap.
- ✓
The penultimate hop pops the top label before forwarding to the egress router
Why this is correct
PHP removes the top label to reduce egress processing.
- ✗
The penultimate hop is disabled by default in MPLS networks
Why it's wrong here
PHP is enabled by default in most implementations.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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