mediumMultiple Select
MPLS Label Operations: Push, Swap, Pop, and Penultimate Hop Popping
Which two statements about MPLS label operations are true? (Choose two.)
Quick Answer
The correct answer is that the egress LSR removes the label before forwarding the IP packet, because in MPLS label operations, the egress router performs a pop to strip the final label, leaving only the native IP packet for onward routing. This contrasts with the ingress LSR, which performs a push to impose the initial label, and intermediate LSRs that execute a swap—replacing the incoming label with a new outgoing label. On the ENCOR 350-401 exam, this topic tests your understanding of the label lifecycle across the MPLS domain, often appearing in multiple-choice questions that require distinguishing which router performs which operation. A common trap is confusing the egress pop with the penultimate hop popping (PHP) feature, where the second-to-last router pops the label so the egress receives a pure IP packet. Remember the mnemonic: Ingress Pushes, Transit Swaps, Egress Pops—and PHP lets the penultimate do the pop early.
⚠ Common exam trap
The trap is mixing up which LSR performs which label operation — candidates often think transit or egress LSRs impose labels, but only the ingress LSR pushes the initial label, while transit swaps and egress pops.
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 LSR imposes a label onto the packet.
Option A is correct because the ingress LSR (label edge router) is the entry point of the MPLS domain and performs label imposition (push), adding an MPLS shim header to the incoming IP packet before forwarding it into the LSP. Option B is correct because the egress LSR (also an LER) performs label disposition (pop), removing the MPLS label so the packet leaves the domain as a normal IP packet toward its destination. Option C is wrong because pushing a new label is an ingress/transit function, not something the egress LSR does when exiting the MPLS domain. Option D is wrong because label imposition occurs only at the ingress edge, not at every LSR along the LSP. Option E is wrong because transit LSRs perform label swapping (swap), not imposition, forwarding packets based on the incoming label.
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 LSR imposes a label onto the packet.
Why this is correct
The ingress LSR classifies the incoming IP packet, assigns the appropriate forwarding equivalence class, and pushes the corresponding label onto the packet before forwarding it into the MPLS domain. This imposition step satisfies the stem's ingress label-operation statement.
- ✓
The egress LSR removes the label before forwarding the IP packet.
Why this is correct
At the egress LSR, the label stack is popped and the packet forwarded as native IP toward the destination, since the next hop lies outside the MPLS domain. This is PHP or ultimate-hop popping, satisfying the stem's egress label-operation statement.
- ✗
The egress LSR pushes a new label onto the packet.
Why it's wrong here
The egress LSR removes the label (penultimate hop popping or final pop) before forwarding to the destination, so it cannot push one. Label pushing belongs to the ingress LSR at the start of the LSP. The option tempts candidates who confuse the two ends of the forwarding path.
- ✗
Each LSR in the LSP performs label imposition.
Why it's wrong here
Only the ingress LSR imposes a label; each subsequent LSR swaps it, and the egress LSR pops it. Claiming every LSR imposes labels misstates the per-hop operation. The option tempts those who generalise label operations across the entire LSP rather than distinguishing ingress, transit and egress roles.
- ✗
Transit LSRs perform label imposition.
Why it's wrong here
Transit LSRs swap the incoming label for the outgoing label; imposition happens only at the ingress LSR. The term 'imposition' specifically means adding the initial label stack entry, so transit devices cannot perform it. Candidates may wrongly assume every LSR along the path adds labels.
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Same concept, more angles
1 more way this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which two statements about MPLS label operations are true? (Choose two.)
medium- ✓ A.The ingress LSR performs a label push operation.
- ✓ B.Transit LSRs perform a label swap operation.
- C.The egress LSR performs a label push operation.
- D.PHP (Penultimate Hop Popping) causes the egress LSR to pop the label.
- E.The egress LSR always performs an IP lookup after label removal.
Why A: In MPLS, the ingress LSR pushes a label onto the packet, and transit LSRs swap the incoming label with an outgoing label based on the LFIB. PHP removes the label before the egress LSR, so the egress LSR does not perform a label lookup. The egress LSR forwards based on the IP header after label removal.
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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