- A
The route-target import on the remote PE is missing the customer's route-target.
Why wrong: If route-target import were missing, the route would not be present in the VRF at all, causing 100% failure.
- B
The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing.
Per-packet load balancing can lead to packets being sent to different PEs, potentially exiting via different remote PEs and causing return packets to arrive out of order or be dropped due to stateful inspection.
- C
The BGP timers are misconfigured, causing the session to flap.
Why wrong: BGP timer misconfiguration would cause session flap, not 50% packet loss on a stable session.
- D
The MTU on the CE-PE links is mismatched.
Why wrong: MTU mismatch typically causes packet drops for large packets, not intermittent connectivity for all traffic.
Quick Answer
The answer is per-packet load balancing across the dual-homed CE routers, which causes asymmetric routing and intermittent failure. When a CE router receives two equal-cost paths from two different PEs for the same MPLS L3VPN prefix, it may distribute individual packets across both links without regard to flow state, sending some packets to one PE and others to the second PE. This per-packet behavior breaks the expected symmetric path for a single session, leading to out-of-order delivery and dropped pings roughly 50% of the time. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of how dual-homed CE designs interact with MPLS VPN load balancing—a common trap is assuming BGP timers or MTU issues are at fault when the real culprit is per-packet distribution. Remember the key insight: equal-cost paths from two PEs do not guarantee symmetric routing; the CE must use per-flow load balancing to avoid this failure. Memory tip: “Per-packet paths produce ping problems; per-flow fixes the flapping.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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 service provider offers L3VPN services to multiple enterprise customers. One customer reports that they cannot reach some remote sites intermittently. The network uses MPLS L3VPN with MP-BGP for VPN route exchange. The PE routers are configured with route-target import and export. The customer's CE router is dual-homed to two different PEs in the same point of presence. The engineer checks the BGP table on both PEs and sees the customer routes with the correct route-target. However, pings from the CE to a remote site fail about 50% of the time, and the flapping pattern suggests load balancing issues. The engineer discovers that the remote site's network prefix is being advertised from both PEs with the same route-target but with different next-hops. The CE has equal-cost paths via both PEs. What is the most likely cause of the intermittent connectivity?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing.
Option C is correct because when a CE receives two equal-cost paths from two PEs, the CE may perform per-packet load balancing, which can cause out-of-order packets and asymmetric routing, leading to failures. Option A is wrong because BGP timers would cause immediate session drops, not 50% failure. Option B is wrong because MTU mismatch would cause consistent failures, not intermittent. Option D is wrong because route-target mismatch would prevent routes from being learned, not cause intermittent connectivity.
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 route-target import on the remote PE is missing the customer's route-target.
Why it's wrong here
If route-target import were missing, the route would not be present in the VRF at all, causing 100% failure.
- ✓
The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing.
Why this is correct
Per-packet load balancing can lead to packets being sent to different PEs, potentially exiting via different remote PEs and causing return packets to arrive out of order or be dropped due to stateful inspection.
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.
- ✗
The BGP timers are misconfigured, causing the session to flap.
Why it's wrong here
BGP timer misconfiguration would cause session flap, not 50% packet loss on a stable session.
- ✗
The MTU on the CE-PE links is mismatched.
Why it's wrong here
MTU mismatch typically causes packet drops for large packets, not intermittent connectivity for all traffic.
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.
What to study next
Got this wrong? Here's your next step.
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 350-501 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Architecture — This question tests Architecture — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing. — Option C is correct because when a CE receives two equal-cost paths from two PEs, the CE may perform per-packet load balancing, which can cause out-of-order packets and asymmetric routing, leading to failures. Option A is wrong because BGP timers would cause immediate session drops, not 50% failure. Option B is wrong because MTU mismatch would cause consistent failures, not intermittent. Option D is wrong because route-target mismatch would prevent routes from being learned, not cause intermittent connectivity.
What should I do if I get this 350-501 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 350-501 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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Last reviewed: Jun 24, 2026
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