What Happens to Untagged Outgoing Labels on an MPLS Router?
What action is performed for untagged outgoing labels in an MPLS router?
Community Votes
81% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The question tests the Untagged/No Label forwarding action in an MPLS LFIB, and the trap is assuming that the LSR performing the untagged operation — rather than the downstream router — does the FIB lookup on the resulting IP packet.
When an MPLS router's LFIB lists an outgoing label as Untagged (No Label), the LSR removes the entire label stack and forwards the resulting native IP packet to the next-hop router. This page confirms that option C — converting the MPLS packet to an IP packet and forwarding it onward — is the correct answer for the untagged outgoing label scenario.
Many candidates choose B, believing the LSR itself converts the MPLS packet to untagged and then performs a FIB lookup; in reality the LSR consults its LFIB, strips the label stack, and the receiving router does the IP FIB lookup.
Community Discussion (7 comments)
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Expert Analysis
Why the Answer Is Correct
On an MPLS LSR, an LFIB entry whose outgoing label is "Untagged" (Cisco also displays it as "No Label") tells the router to remove the entire MPLS label stack, producing a native IP packet, and forward that packet to the next-hop router. This is exactly the behavior of penultimate hop popping (PHP), where the penultimate LSR pops the label so the egress LSR receives a plain IP packet. The question asks what action the MPLS router performs for an untagged outgoing label, and that action is converting the incoming MPLS packet into an IP packet followed by forwarding it to the next router, which option C states precisely. As Horsefeathers described it, the operation means to "strip all of the MPLS labels off the packet and forward the raw IP packet". The FIB lookup then occurs at the downstream router, not at the LSR that performed the untagged operation.
Why the Other Options Are Wrong
Options A and B misplace the lookup: the router performing the untagged operation relies on its LFIB to decide the forwarding action, and it is the next router receiving the plain IP packet that consults its FIB (not the RIB) to route it further. Option D reverses the concept — an untagged outgoing label means removing labels, whereas imposing a label on an untagged packet describes label imposition at an ingress LSR. Option C is the only choice that matches the definition of the Untagged/No Label operation on an LSR. Bombbear_W argued for B by claiming the local router performs a FIB lookup, but that description actually belongs to the downstream router that receives the untagged packet.
Community Comment Notes
The majority of learners (Horsefeathers, Pietjeplukgeluk, dapardo, d740f62 and XBfoundX) selected C, and their reasoning matches the vendor explanation that untagged means stripping all labels. Pietjeplukgeluk stated "if outbound need to be send untagged, the MPLS part is stripped off", and dapardo highlighted the key phrase "untagged outgoing labels" as the deciding clue. The minority (Bombbear_W and Gesti) chose B, conflating the downstream IP lookup with the LSR's own LFIB-based label operation. That consensus, combined with MPLS forwarding doctrine, supports C.
Exam Strategy
Memorize the difference between Untagged and Pop Label in show mpls forwarding-table: Untagged removes the whole label stack and forwards a native IP packet, while Pop Label removes only the top label and keeps the remaining stack. Also remember that the LFIB drives the local action and the downstream router performs the IP FIB lookup.
Frequently Asked Questions
Why is option B wrong for untagged outgoing labels in MPLS?
The LSR performing the untagged operation uses its LFIB to decide the action and forwards a plain IP packet; the downstream router, not the LSR itself, then performs the FIB lookup.
What is the difference between Untagged and Pop Label in an MPLS LFIB?
Untagged (No Label) removes the whole label stack and forwards a native IP packet, while Pop Label removes only the top label and forwards the remaining labeled packet.
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