350-501 Networking Practice Question
An engineer is troubleshooting MPLS forwarding. On a router, the 'show mpls forwarding-table' command displays that for a specific FEC, the outgoing label is 'Untagged'. What does this indicate?
⚠ Common exam trap
Many candidates confuse 'Untagged' with the egress LSR behavior or with an implicit null label, but Cisco specifically uses 'Untagged' to denote the penultimate hop's PHP action, not the egress router's label disposition.
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 router is the penultimate hop, performing PHP.
When the 'show mpls forwarding-table' command shows 'Untagged' for a specific FEC, it indicates that the router is performing Penultimate Hop Popping (PHP). In PHP, the penultimate router (the router before the egress LSR) pops the MPLS label and forwards the packet as an IP packet (untagged) to the egress LSR. This is standard behavior to avoid the egress LSR having to perform a double lookup.
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 label has been explicitly null.
Why it's wrong here
Incorrect. Explicit null would be shown as 'ExpNull'.
- ✓
The router is the penultimate hop, performing PHP.
Why this is correct
Correct. 'Untagged' means the label is removed before forwarding.
- ✗
The router is the egress LSR.
Why it's wrong here
Incorrect. The egress LSR would have a different label indication.
- ✗
The next-hop router does not support MPLS.
Why it's wrong here
Incorrect. This is not the standard interpretation of 'Untagged'.
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Written by Johnson Ajibi, MSc IT Security
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