350-501 MPLS and Segment Routing Practice Question
Exhibit
PE1# show mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16 Pop 10.1.1.0/24 0 Gi0/0/0 10.0.0.2 17 19 10.2.2.0/24 1234 Gi0/0/0 10.0.0.2 18 Untagged 10.3.3.0/24 0 Gi0/0/0 10.0.0.2
Refer to the exhibit. An engineer is troubleshooting packet loss in the MPLS core. For prefix 10.3.3.0/24, the outgoing label is 'Untagged'. What does this indicate?
⚠ Common exam trap
Cisco often tests the distinction between 'Untagged' (meaning no label is imposed, typically due to PHP with implicit null) and 'Pop' (which explicitly indicates the label is removed), leading candidates to confuse the two or incorrectly associate 'Untagged' with unreachability.
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
✓
No MPLS label is being imposed on outgoing packets for this prefix
When the outgoing label for a prefix in the MPLS forwarding table shows 'Untagged', it means that for packets destined to that prefix, no MPLS label is imposed. The router will forward the packet as a standard IP packet (without an MPLS header) out the egress interface. This typically occurs when the next-hop router has signaled an implicit null label (label 3) via LDP, instructing the upstream router to pop the label stack and send the packet as IP.
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 packet will have an implicit null label (label 3) imposed
Why it's wrong here
Implicit null is indicated by 'Pop'.
- ✗
The prefix is not reachable and packets will be dropped
Why it's wrong here
Packets may be forwarded without label, not necessarily dropped.
- ✗
The outgoing label is the same as the local label (label 18)
Why it's wrong here
Local and outgoing labels are different.
- ✓
No MPLS label is being imposed on outgoing packets for this prefix
Why this is correct
Untagged means no label.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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