Click the Exhibit button.
Referring to the exhibit, a working L3VPN exists that connects VPN-A sites. CoS is configured correctly to match on the MPLS EXP bits of the LSP. but when traffic is sent from Site-1 to Site-2. PE-2 is not classifying the traffic correctly.
What should you do to solve the problem?
To solve the problem where PE-2 is not classifying the traffic correctly based on the MPLS EXP bits, you need to configure the explicit-null statement on PE-2. Configuring explicit-null on PE-2 allows the MPLS EXP bits to be preserved when the MPLS header is removed, ensuring that the CoS information is correctly interpreted by PE-2.
Ultimate Hop Popping (UHP) must be configured on the egress LSR to keep the EXP bit along the LSP. So configuration is needed on PE-2 in this case.
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.
Explicit-null should be configured on PE-2
C is correct
The QoS is on the label and the second-to-last router on the LSP should keep the label on instead of having it arrived popped
• By default, PHP (Penultimate Hop Popping) removes the top MPLS label at the penultimate router (P-1), before the packet reaches PE-2. • This causes loss of EXP bits from the outer label, preventing PE-2 from classifying the traffic based on them. • Using explicit-null on PE-1 ensures the outer MPLS label is preserved, but with label value 0 (IPv4 explicit-null), so that EXP bits reach PE-2. • PE-2 can then read the EXP bits and classify the traffic correctly