An election-office engineer packet-captures near an egress PE and notices the top transport label is already gone one hop before that PE. Which MPLS behavior most likely explains the missing top label?
Select an answer to reveal the explanation.
Short Explanation
PHP is the neighbor before the finish line tearing off the outer shipping label so the egress PE does not have to. Capture one hop early and the top label looks "missing" — that is often normal, not a broken LSP. Do not confuse it with CE tricks or QoS remarking deleting labels.
Full Explanation
Penultimate hop popping lets the router before the egress PE pop the outer transport label, so the egress PE receives an unlabeled IP packet or a remaining VPN label without an extra transport-label lookup. An election-office capture taken just upstream of egress often shows no top label even when the LSP is healthy—PHP explains that “missing” outer label.
Ultimate hop popping on the CE removing VPN labels before VRF entry is not the usual explanation for a missing top transport label near the egress PE. LDP session failure silently stripping all labels across the entire LSP would be a fault, not the normal near-egress pattern. EXP-to-DSCP remarking does not delete the label stack whenever voice packets are present.
Exam caveat: account for PHP when interpreting near-egress captures; absence of the outer label one hop early is often expected behavior.
Operational check: traceroute the LSP, identify the penultimate hop, confirm PHP is enabled as designed, and verify the egress PE still correctly forwards into the election VRF using the remaining label or IP lookup. For civic operations, record the chosen control and the rejected alternatives in the change record so future on-call engineers understand which failure modes were avoided for citizen-facing services.