300-410 · Question #8
Refer to the following output: Router# show tag-switching tdp bindings (...) tib entry: 10.10.10.1/32, rev 31 local binding: tag: 18 remote binding: tsr: 10.10.10.1:0, tag: imp-null remote binding…
The correct answer is D. Pop the label. In MPLS, 'imp-null' (implicit null) is a special label indicating that the penultimate hop router should pop the MPLS label, forwarding a native IP packet to the egress router.
Question
Exhibit
Options
- AInclude the EXP bit
- BExclude the EXP bit
- CImpove the label
- DPop the label
How the community answered
(51 responses)- A6% (3)
- B2% (1)
- C2% (1)
- D90% (46)
Why each option
In MPLS, 'imp-null' (implicit null) is a special label indicating that the penultimate hop router should pop the MPLS label, forwarding a native IP packet to the egress router.
`EXP bit` (Experimental bits) in the MPLS header is used for QoS, and 'imp-null' has no direct implication on its inclusion.
`EXP bit` (Experimental bits) in the MPLS header is used for QoS, and 'imp-null' has no direct implication on its exclusion.
`Impove the label` is not a recognized term or function in MPLS.
'imp-null', or implicit null, is a special MPLS label (value 0 for IPv4) that signals the upstream router (the penultimate hop) to remove or 'pop' the MPLS label from the packet. The packet is then forwarded as a native IP packet to the egress Label Edge Router (LER), which performs the final IP lookup; this mechanism is known as Penultimate Hop Popping (PHP).
Concept tested: MPLS Implicit Null label (PHP)
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_te_fwd/configuration/15-mt/mp-te-fwd-15-mt-book/mp-te-fwd-oview.html
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