350-501 MPLS and Segment Routing Practice Question
A service provider has a network with PE1, P1, P2, and PE2 connected in a chain: PE1-P1-P2-PE2. The IGP is IS-IS with wide metrics, and MPLS LDP is enabled on all interfaces. The goal is to provide L3VPN services between PE1 and PE2. The engineer has configured MP-iBGP between PE1 and PE2, and the VPNv4 routes are exchanged. However, when a CE router behind PE1 tries to reach the CE behind PE2, the traffic fails. The engineer checks the MPLS forwarding table on PE1 and sees that the label for the BGP next-hop (PE2's loopback) is 'Untagged'. The BGP next-hop is reachable via ICMP. What is the most likely cause of this issue?
⚠ Common exam trap
Cisco often tests the distinction between BGP route exchange (which works) and MPLS label assignment (which fails), leading candidates to incorrectly focus on routing protocol issues or MTU/TTL parameters instead of verifying LDP adjacency and label bindings.
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 between PE1 and P1.
The MPLS forwarding table on PE1 shows the label for PE2's loopback as 'Untagged', which indicates that LDP has not assigned a label for that prefix on the incoming interface. Since LDP is responsible for distributing labels for IGP routes (like loopbacks) in an MPLS LDP-enabled network, the missing label means LDP is not operational on the link between PE1 and P1. Without a label, PE1 cannot push an MPLS label for the BGP next-hop, causing the VPNv4 traffic to be dropped or forwarded as IP, which fails because the core routers (P1, P2) do not have the VPN route.
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 IS-IS metric between PE1 and P1 is too high, causing suboptimal routing.
Why it's wrong here
High metric might affect path, but not label assignment.
- ✗
The MPLS TTL propagation is disabled, causing packets to be dropped.
Why it's wrong here
TTL propagation does not affect label binding.
- ✓
LDP is not enabled on the interface between PE1 and P1.
Why this is correct
Without LDP, no label is received from P1 for the BGP next-hop.
- ✗
The MTU on the link between PE1 and P1 is set to 1500, causing fragmentation.
Why it's wrong here
Fragmentation does not cause 'Untagged'.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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