Why MPLS Forwarding Table Shows Untagged
Exhibit
Refer to the exhibit. PE1# show mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16 Pop Label 10.1.1.0/24 0 Gi0/0/0 192.168.1.1 17 18 10.2.2.0/24 0 Gi0/0/1 192.168.2.2 18 19 10.3.3.0/24 0 Gi0/0/1 192.168.2.2 19 Untagged 10.4.4.0/24 0 Gi0/0/2 192.168.3.3
Refer to the exhibit. A network engineer notices that the local label 19 for prefix 10.4.4.0/24 shows 'Untagged' in the outgoing label column. What is the most likely cause?
Quick Answer
The most likely cause is that LDP is not enabled on the interface toward 192.168.3.3. When LDP is disabled on the outgoing interface, the router cannot establish a label-switched path with the next-hop neighbor, so it never receives a label binding for the prefix 10.4.4.0/24. Without an incoming label-to-outgoing label mapping, the MPLS forwarding table shows 'Untagged' in the outgoing label column, meaning the router must pop the label and forward the packet as a standard IP packet. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS label distribution process and the relationship between the LIB and LFIB. A common trap is assuming 'Untagged' indicates a misconfiguration of the local label itself, when in fact the issue is the missing LDP session on the egress interface. Memory tip: no LDP neighbor, no label swap—only a pop.
⚠ Common exam trap
Cisco often tests the distinction between 'Untagged' (no label from LDP neighbor) and 'Pop tag' (implicit-null from PHP), leading candidates to mistakenly attribute 'Untagged' to PHP when it actually indicates a missing LDP adjacency.
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
✓
LDP is not enabled on the interface toward 192.168.3.3.
When LDP is not enabled on the interface toward the next-hop router (192.168.3.3), the local router cannot exchange label bindings with that neighbor. As a result, the local label for prefix 10.4.4.0/24 remains in the LIB but is not programmed into the LFIB with an outgoing label, showing 'Untagged' because the router must pop the label (or forward as IP) when sending traffic to that next hop.
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 next-hop router 192.168.3.3 is unreachable.
Why it's wrong here
If unreachable, the entry would not be in the LFIB.
- ✗
Penultimate Hop Popping is enabled on the next-hop router.
Why it's wrong here
PHP would show 'Pop Label', not 'Untagged'.
- ✗
The MTU on interface Gi0/0/2 is too small.
Why it's wrong here
MTU does not affect label assignment.
- ✓
LDP is not enabled on the interface toward 192.168.3.3.
Why this is correct
If LDP is not enabled, no label is received from the next-hop, resulting in 'Untagged'.
Go deeper
Related to this question
About these practice questions
Courseiva writes every 350-501 question from scratch — 971 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way this is tested on 350-501
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. An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?
easy- A.The remote PE is configured with a different VPN ID.
- B.The local PE does not have a route to its own loopback.
- ✓ C.LDP is not enabled on the core-facing interfaces.
- D.LDP is not enabled on the PE-CE interface.
Why C: MPLS VPN traffic relies on LDP to distribute labels for the BGP next-hop loopback addresses used in the VPNv4 route exchange. If LDP is not enabled on the core-facing interfaces, the local PE cannot obtain a label binding for the remote PE's loopback, causing the traffic to be forwarded via IP (without a label) or taking a suboptimal path. Enabling LDP on all core-facing interfaces resolves the missing label binding and restores optimal label-switched forwarding.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.