350-501 MPLS and Segment Routing Practice Question
An engineer is troubleshooting an MPLS L3VPN where customers behind CE1 cannot reach a specific prefix behind CE2. The PE routers are using OSPF as the IGP and LDP for label distribution. On PE2, the prefix is present in the VRF routing table, but not in the VRF forwarding table. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between the routing table (RIB) and the forwarding table (FIB) in MPLS VPNs, trapping candidates who assume that a route present in the RIB automatically means it is usable for forwarding, when in fact the FIB requires a valid label binding to install the route.
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 route is missing a label in the LFIB on PE2.
The prefix is present in the VRF routing table (RIB) but missing from the VRF forwarding table (FIB) on PE2. This indicates that the route has been learned via BGP and installed in the RIB, but the MPLS VPN label (the inner label) required to forward the packet across the MPLS core is absent. Without a valid label in the LFIB, the CEF (FIB) cannot install the route, causing the reachability failure. Option D correctly identifies this missing label in the LFIB as the root cause.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
MTU mismatch is causing the VPN label to be dropped.
Why it's wrong here
MTU mismatch would cause packet drops, not prevent the route from being installed in the forwarding table.
- ✗
OSPF is not redistributing the BGP routes into the IGP on PE2.
Why it's wrong here
OSPF redistribution is not required for MPLS label allocation; labels are assigned via LDP.
- ✗
The VRF is not properly configured on PE2's interface toward CE2.
Why it's wrong here
If the VRF were misconfigured, the route would not appear in the VRF routing table either.
- ✓
The route is missing a label in the LFIB on PE2.
Why this is correct
If the label is missing, the route cannot be installed in the VRF forwarding table.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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