350-501 MPLS and Segment Routing Practice Question
Exhibit
Refer to the exhibit. The output below is from a Cisco router. R1# show mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16 27 10.1.1.0/24 0 Gi0/0 172.16.1.2 17 31 10.2.2.0/24 0 Gi0/0 172.16.1.2 18 Pop Label 10.3.3.0/24 0 Gi0/1 172.16.2.2 19 No Label 10.4.4.0/24 no vif Gi0/2 172.16.3.2 20 35 10.5.5.0/24 0 Gi0/0 172.16.1.2
Based on the exhibit, which prefix is missing a label binding and likely causing traffic to be dropped?
⚠ Common exam trap
Cisco often tests the concept that a missing local label in the LDP binding table indicates the prefix is not reachable via LDP, and candidates may mistakenly focus on remote labels or assume a label of '0' or 'implicit null' means a problem, when the real issue is the absence of a local label.
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
✓
10.4.4.0/24
The exhibit shows an MPLS LDP label binding table. Prefix 10.4.4.0/24 has no local label assigned (shown as 'no label' or missing), meaning LDP has not established a label binding for this FEC. Without a label, traffic destined for 10.4.4.0/24 cannot be label-switched and will be dropped or forwarded via IP lookup (if enabled), but in an MPLS core, this typically results in packet loss.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
10.2.2.0/24
Why it's wrong here
This prefix has an outgoing label (31), so it is working.
- ✗
10.1.1.0/24
Why it's wrong here
This prefix has an outgoing label (27), so it is working.
- ✓
10.4.4.0/24
Why this is correct
'No Label' means no label binding exists; traffic to this prefix will be dropped.
- ✗
10.3.3.0/24
Why it's wrong here
'Pop Label' is a valid operation (PHP), so traffic can still be forwarded.
- ✗
10.5.5.0/24
Why it's wrong here
This prefix has an outgoing label (35), so it is working.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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