350-501 MPLS and Segment Routing Practice Question
A service provider is troubleshooting an MPLS L3VPN scenario where a customer in VRF Blue cannot reach a server in VRF Blue at a remote site. The PEs are running MPLS with LDP. The VRF on both PEs shows the remote prefix. The PE at the local site shows the label from the remote PE for the prefix in the BGP table. However, when pinging from the CE, the packets are dropped. A packet capture on the core shows MPLS packets with the correct VPN label, but the transport label is missing. Further investigation shows that the LDP session between the two PEs is up, but the LDP label binding for the remote PE's loopback is not present. What is the most likely cause and correct action?
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
✓
Check MPLS LDP interface configuration and enable LDP on all core interfaces
The LDP session is up, but the label binding for the remote loopback is missing. This could be because LDP is not enabled on the underlying interfaces between the PEs, or because the label space is configured incorrectly. Typically, if LDP is enabled on all core interfaces, label bindings for loopbacks are automatically exchanged. The missing binding suggests that either LDP is not configured on some interfaces, or an access-list is blocking LDP. The most direct action is to verify that LDP is enabled on all core interfaces using the command 'show mpls ldp interface'.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the TTL on the MPLS packets
Why it's wrong here
TTL issues would not cause missing label binding.
- ✗
Change the transport address in LDP to the loopback IP
Why it's wrong here
Transport address is already the loopback by default; this is not a missing binding issue.
- ✗
Configure the remote PE's loopback to be advertised via BGP
Why it's wrong here
The loopback is already in the IGP; LDP uses IGP to distribute labels.
- ✓
Check MPLS LDP interface configuration and enable LDP on all core interfaces
Why this is correct
LDP must be enabled on each core interface to exchange label bindings for loopbacks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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