Question 250 A provider edge learns an MPLS label for a customer prefix, but an engineer expects every core hop to inspect the customer's destination IP at each transition. Which technical mechanism best explains the behavior observed in this scenario? LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. Implicit-null label advertisement can request removal of the transport label at the penultimate hop to reduce egress lookup work. A complete LSP needs continuous label switching across the intended MPLS-capable path. ExplanationCorrect Answer B:MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. This explains why the initial condition produces the observed failure. The direct repair is: Trace the label-switched path and forwarding-label entries instead of assuming repeated customer-IP lookups in the core. Validate through: Compare incoming and outgoing labels at intermediate routers and observe delivery at the final egress. Other Answers A:LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. C:Implicit-null label advertisement can request removal of the transport label at the penultimate hop to reduce egress lookup work. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. D:A complete LSP needs continuous label switching across the intended MPLS-capable path. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries.
Question 251 A provider edge learns an MPLS label for a customer prefix, but an engineer expects every core hop to inspect the customer's destination IP at each transition. After applying the correction, which verification evidence most directly demonstrates that this issue is resolved? Capture or inspect LFIB pop operation at the penultimate node and successful delivery to egress. Verify FEC reachability in the underlay and the corresponding LDP label binding. Compare incoming and outgoing labels at intermediate routers and observe delivery at the final egress. Trace outer transport forwarding and the distinct customer-specific forwarding context at ingress and egress. ExplanationCorrect Answer C:Compare incoming and outgoing labels at intermediate routers and observe delivery at the final egress. This checks the affected traffic or control-plane behavior, not merely a generic up/down indicator. The underlying mechanism is: MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. Other Answers A:Capture or inspect LFIB pop operation at the penultimate node and successful delivery to egress. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. B:Verify FEC reachability in the underlay and the corresponding LDP label binding. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. D:Trace outer transport forwarding and the distinct customer-specific forwarding context at ingress and egress. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries.
Question 252 A provider edge learns an MPLS label for a customer prefix, but an engineer expects every core hop to inspect the customer's destination IP at each transition. Which lasting design or operational safeguard best prevents this particular issue from recurring? Require labeled-service path tests in every backbone circuit acceptance plan. Monitor label distribution and LFIB consistency whenever core interfaces or protocols change. Review label path impact when making IGP cost engineering changes. Document the label stack and per-hop switching behavior in provider path troubleshooting. ExplanationCorrect Answer D:Document the label stack and per-hop switching behavior in provider path troubleshooting. This addresses the recurring cause: MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. A targeted acceptance check is: Compare incoming and outgoing labels at intermediate routers and observe delivery at the final egress. Other Answers A:Require labeled-service path tests in every backbone circuit acceptance plan. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. B:Monitor label distribution and LFIB consistency whenever core interfaces or protocols change. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. C:Review label path impact when making IGP cost engineering changes. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries.
Question 253 A packet arrives at a core router with an outer label that has no matching forwarding entry; the router drops it even though an IP route exists. Which corrective action most directly resolves the documented failure without suppressing unrelated safeguards? Investigate the missing LFIB label binding or LSP continuity before changing customer IP routes. Restore routability and required LDP discovery/transport reachability to the peer's label-space identifier. Separate transport label delivery from VPN service isolation and validate the applicable VRF/service labels. Validate each router's local label binding and adjacency rather than demanding one network-wide numeric label. ExplanationCorrect Answer A:Investigate the missing LFIB label binding or LSP continuity before changing customer IP routes. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. Inspect the assigned incoming label and associated label operation on the failing LSR. Other Answers B:Restore routability and required LDP discovery/transport reachability to the peer's label-space identifier. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. C:Separate transport label delivery from VPN service isolation and validate the applicable VRF/service labels. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. D:Validate each router's local label binding and adjacency rather than demanding one network-wide numeric label. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it.
Question 254 A packet arrives at a core router with an outer label that has no matching forwarding entry; the router drops it even though an IP route exists. Which technical mechanism best explains the behavior observed in this scenario? A complete LSP needs continuous label switching across the intended MPLS-capable path. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. MPLS labels are locally significant to a label-switching context, not globally unique identifiers for all routers. MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. ExplanationCorrect Answer B:A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. This explains why the initial condition produces the observed failure. The direct repair is: Investigate the missing LFIB label binding or LSP continuity before changing customer IP routes. Validate through: Inspect the assigned incoming label and associated label operation on the failing LSR. Other Answers A:A complete LSP needs continuous label switching across the intended MPLS-capable path. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. C:MPLS labels are locally significant to a label-switching context, not globally unique identifiers for all routers. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. D:MPLS core label-switching routers forward based on labels using swap, push or pop operations according to LFIB entries. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it.
Question 255 A packet arrives at a core router with an outer label that has no matching forwarding entry; the router drops it even though an IP route exists. After applying the correction, which verification evidence most directly demonstrates that this issue is resolved? Inspect each hop's local incoming/outgoing label mapping and verify end-to-end continuity. Check LDP neighbors and the transport address reachability after correcting underlay routing. Inspect the assigned incoming label and associated label operation on the failing LSR. Compare incoming and outgoing labels at intermediate routers and observe delivery at the final egress. ExplanationCorrect Answer C:Inspect the assigned incoming label and associated label operation on the failing LSR. This checks the affected traffic or control-plane behavior, not merely a generic up/down indicator. The underlying mechanism is: A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. Other Answers A:Inspect each hop's local incoming/outgoing label mapping and verify end-to-end continuity. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. B:Check LDP neighbors and the transport address reachability after correcting underlay routing. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. D:Compare incoming and outgoing labels at intermediate routers and observe delivery at the final egress. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it.
Question 256 A packet arrives at a core router with an outer label that has no matching forwarding entry; the router drops it even though an IP route exists. Which lasting design or operational safeguard best prevents this particular issue from recurring? Review label path impact when making IGP cost engineering changes. Gate MPLS provisioning on complete underlay IGP and loopback visibility. Document transport-versus-service labels in carrier design reviews. Monitor label distribution and LFIB consistency whenever core interfaces or protocols change. ExplanationCorrect Answer D:Monitor label distribution and LFIB consistency whenever core interfaces or protocols change. This addresses the recurring cause: A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. A targeted acceptance check is: Inspect the assigned incoming label and associated label operation on the failing LSR. Other Answers A:Review label path impact when making IGP cost engineering changes. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. B:Gate MPLS provisioning on complete underlay IGP and loopback visibility. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. C:Document transport-versus-service labels in carrier design reviews. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it.
Question 257 An LDP adjacency forms over a new interface but labels are absent for a subset of transit FECs because the underlay has no route to those FECs. Which corrective action most directly resolves the documented failure without suppressing unrelated safeguards? Repair IGP reachability to the required loopback and FEC before troubleshooting service labels. Enable or repair the supported MPLS forwarding and label-distribution configuration on the missing core link. Trace FEC next-hop selection in the underlay and align the core routing metric with the desired path. Recognize penultimate-hop popping and verify that the egress receives the intended remaining header or payload. ExplanationCorrect Answer A:Repair IGP reachability to the required loopback and FEC before troubleshooting service labels. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. Verify FEC reachability in the underlay and the corresponding LDP label binding. Other Answers B:Enable or repair the supported MPLS forwarding and label-distribution configuration on the missing core link. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. C:Trace FEC next-hop selection in the underlay and align the core routing metric with the desired path. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. D:Recognize penultimate-hop popping and verify that the egress receives the intended remaining header or payload. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability.
Question 258 An LDP adjacency forms over a new interface but labels are absent for a subset of transit FECs because the underlay has no route to those FECs. Which technical mechanism best explains the behavior observed in this scenario? Implicit-null label advertisement can request removal of the transport label at the penultimate hop to reduce egress lookup work. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. Label forwarding for a FEC commonly follows the underlay next-hop decision, so IGP metric changes can alter the LSP. ExplanationCorrect Answer B:LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. This explains why the initial condition produces the observed failure. The direct repair is: Repair IGP reachability to the required loopback and FEC before troubleshooting service labels. Validate through: Verify FEC reachability in the underlay and the corresponding LDP label binding. Other Answers A:Implicit-null label advertisement can request removal of the transport label at the penultimate hop to reduce egress lookup work. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. C:A labeled packet requires a usable incoming-label forwarding entry; an unrelated global IP route does not replace it. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. D:Label forwarding for a FEC commonly follows the underlay next-hop decision, so IGP metric changes can alter the LSP. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability.
Question 259 An LDP adjacency forms over a new interface but labels are absent for a subset of transit FECs because the underlay has no route to those FECs. After applying the correction, which verification evidence most directly demonstrates that this issue is resolved? Check LDP neighbors and the transport address reachability after correcting underlay routing. Compare IGP next hops and LFIB outgoing adjacency before and after the metric correction. Verify FEC reachability in the underlay and the corresponding LDP label binding. Check per-hop MPLS interface state, LDP adjacency and LFIB entries across the whole path. ExplanationCorrect Answer C:Verify FEC reachability in the underlay and the corresponding LDP label binding. This checks the affected traffic or control-plane behavior, not merely a generic up/down indicator. The underlying mechanism is: LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. Other Answers A:Check LDP neighbors and the transport address reachability after correcting underlay routing. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. B:Compare IGP next hops and LFIB outgoing adjacency before and after the metric correction. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. D:Check per-hop MPLS interface state, LDP adjacency and LFIB entries across the whole path. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability.
Question 260 An LDP adjacency forms over a new interface but labels are absent for a subset of transit FECs because the underlay has no route to those FECs. Which lasting design or operational safeguard best prevents this particular issue from recurring? Review label path impact when making IGP cost engineering changes. Document transport-versus-service labels in carrier design reviews. Document the label stack and per-hop switching behavior in provider path troubleshooting. Gate MPLS provisioning on complete underlay IGP and loopback visibility. ExplanationCorrect Answer D:Gate MPLS provisioning on complete underlay IGP and loopback visibility. This addresses the recurring cause: LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. A targeted acceptance check is: Verify FEC reachability in the underlay and the corresponding LDP label binding. Other Answers A:Review label path impact when making IGP cost engineering changes. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. B:Document transport-versus-service labels in carrier design reviews. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability. C:Document the label stack and per-hop switching behavior in provider path troubleshooting. LDP label bindings rely on reachable forwarding-equivalence classes and usable transport reachability.
