Exam JN0-664 topic 1 question 52 discussion

Option D is Correct: “Configure the explicit-null statement on PE-1.” Here’s why: • Configuring explicit-null on PE-1 (the ingress PE) tells the network to not use PHP. • Instead of popping the top label, P-1 will replace it with label 0 (explicit-null), and send it to PE-2. • This preserves the EXP bits in that top label, so PE-2 can read and classify traffic based on those bits. ⸻ Now what about PE-2 and UHP? You’re correct that PE-2 must be capable of processing that explicit-null label. But: • PE-2 already handles label 0 (explicit-null) by default. • You don’t need to configure explicit-null on PE-2—you just need to ensure PE-1 requests it during label distribution (e.g., via LDP or RSVP). So while you could say UHP behavior is relevant at the egress (PE-2), the trigger to keep the label till PE-2 comes from the ingress PE — hence the config is on PE-1.