A civic data center sees an LDP session down between P routers that still have IGP adjacency. What is the most likely describe-level impact on MPLS forwarding?
Select an answer to reveal the explanation.
Short Explanation
IGP can still be chatting while LDP is silent—roads exist, but nobody handed out the MPLS stickers. Result: IP may route, but labeled LSPs that needed those bindings go dark.
Full Explanation
In a civic data center, if the LDP session between P routers is down while the IGP adjacency remains up, IP routing can continue but MPLS label switching that depended on those label bindings can fail. Without LDP, neighbors do not exchange (or refresh) the bindings needed to program LSPs for IGP FECs, so labeled civic services blackhole or fall back unpredictably even though OSPF/IS-IS still exchanges routes. That split-brain between IGP up and LDP down is the describe-level impact to recognize.
IGP adjacency immediately failing because LDP is required to exchange hellos is wrong. IGP hellos and adjacency are independent of LDP; LDP runs separately (UDP discovery / TCP session) and is not required for IGP keepalives.
All CE routers losing their VRF interfaces until LDP is restarted on the PE overstates the failure. VRF interfaces and PE-CE attachments do not automatically disappear solely because a P-P LDP session is down, though VPN labeled transport across that core segment can break.
IPsec SAs tearing down because MPLS encryption depends on LDP keepalives confuses planes. MPLS is not IPsec, and LDP keepalives do not key IPsec SAs.
Exam caveat: LDP-IGP synchronization exists specifically because IGP-up/LDP-down is dangerous—IGP may forward traffic onto a link before labels exist. ENARSI expects you to separate control protocols: IGP reachability ≠ labeled LSP readiness.
Operational check: show mpls ldp neighbor (down) versus show ip ospf neighbor / isis adjacency (up); show mpls forwarding-table for missing or untagged entries; test labeled vs unlabeled traceroute. Restore LDP (transport-address reachability, mpls ip, authentication) and confirm bindings return before declaring the civic MPLS path healthy.