The answer is that the ingress PE pushes two labels: the outer IGP label and the inner VPN label. This is correct because in MPLS L3VPN, the provider edge router must first encapsulate the customer packet with a VPN label that identifies the specific VRF and destination prefix on the egress PE, then stack an outer IGP label that routes the packet across the MPLS core to that egress PE. The egress PE pops the IGP label and uses the inner VPN label to determine the correct VRF and forward the packet to the appropriate CE. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of the MPLS L3VPN label stack operations, often appearing in questions about label imposition and disposition. A common trap is confusing the direction of label operations—remember that the ingress PE always pushes two labels, while the egress PE pops the outer label and uses the inner VPN label. A helpful memory tip: think of the IGP label as the "envelope" that gets the packet to the right post office (egress PE), and the VPN label as the "room number" that delivers it to the correct tenant (CE).
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which of the following statements about MPLS L3VPN label operations is true?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ingress PE pushes two labels: the outer IGP label and the inner VPN label.
In MPLS L3VPN, the PE router assigns a per-VRF label (VPN label) for each prefix in the VRF. When forwarding a packet from the CE, the ingress PE pushes an IGP label (for the egress PE) and the VPN label. The egress PE pops the IGP label and uses the VPN label to identify the VRF and forward to the correct CE.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The ingress PE pushes two labels: the outer IGP label and the inner VPN label.
Why this is correct
The outer label is used to reach the egress PE, and the inner label identifies the VRF and the specific prefix.
Related concept
Static NAT maps one inside address to one outside address.
✗
The egress PE uses the IGP label to determine the VRF.
Why it's wrong here
The IGP label is used for transport; the VPN label is used for VRF identification.
✗
The P routers swap the VPN label as they forward the packet.
Why it's wrong here
P routers only swap the outer IGP label; they do not look at the VPN label.
✗
The ingress PE pushes only one label (the VPN label) and uses the IP destination for forwarding.
Why it's wrong here
Without an IGP label, the packet would not be label-switched across the core; two labels are needed.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
MPLS L3VPN — This question tests MPLS L3VPN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ingress PE pushes two labels: the outer IGP label and the inner VPN label. — In MPLS L3VPN, the PE router assigns a per-VRF label (VPN label) for each prefix in the VRF. When forwarding a packet from the CE, the ingress PE pushes an IGP label (for the egress PE) and the VPN label. The egress PE pops the IGP label and uses the VPN label to identify the VRF and forward to the correct CE.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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