The answer is a duplicate loopback configuration on PE1, where the PE2 loopback address is accidentally configured as an interface on PE1. This causes the router to believe it is the egress LSR for the BGP next-hop 192.168.1.2, so the MPLS forwarding table shows an aggregate label with no real outgoing interface—meaning the router tries to process the packet locally rather than label-switching it toward PE2, breaking the L3VPN path. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to interpret the show mpls forwarding-table detail output, where aggregate indicates the router sees itself as the destination for that prefix, a common trap when troubleshooting MPLS L3VPN reachability. Remember the memory tip: “Aggregate means I’m the gate—if the loopback’s a duplicate, the traffic won’t propagate.”
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A network engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using eBGP with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 and the output interface as GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label but the outgoing interface is 'aggregate'. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The PE2 loopback address is accidentally configured on PE1.
The label for the BGP next-hop is pointing to 'aggregate', which means the router is the egress LSR for that prefix. This occurs when the PE2 loopback is also configured on PE1, causing the router to think it is the destination. The traffic is then dropped or looped because the router tries to process the packet locally instead of forwarding it.
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 PE2 loopback address is accidentally configured on PE1.
Why this is correct
Correct: If PE1 has the same loopback IP, it will treat itself as the egress for that prefix, causing 'aggregate' in the LFIB.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
LDP is not enabled on the core-facing interfaces.
Why it's wrong here
Incorrect because a label is present, so LDP is working.
✗
The VRF route-target import is misconfigured.
Why it's wrong here
Incorrect because the route is installed in the VRF.
✗
The MP-BGP session is using the wrong update-source.
Why it's wrong here
Incorrect because the route is received and installed.
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 PE2 loopback address is accidentally configured on PE1. — The label for the BGP next-hop is pointing to 'aggregate', which means the router is the egress LSR for that prefix. This occurs when the PE2 loopback is also configured on PE1, causing the router to think it is the destination. The traffic is then dropped or looped because the router tries to process the packet locally instead of forwarding it.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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