Question 958 of 2,152

MPLS L3VPN →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is asymmetric routing causing the return path to use a different interface, which violates the strict uRPF check. In strict mode, uRPF verifies that the source address of an incoming packet has a route in the routing table pointing back to the exact interface on which the packet was received. When traffic from the CE to the PE takes one path but return traffic takes a different path—common in MPLS L3VPNs due to load balancing or policy-based forwarding—the strict check fails and drops legitimate packets, even though the CE has a valid return route. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the critical difference between strict and loose uRPF modes, often appearing as a trick where the engineer assumes a valid route is sufficient. The trap is forgetting that strict mode requires interface symmetry, not just reachability. Memory tip: “Strict is picky—same door in, same door out; loose just needs a way out somewhere.”

300-410 MPLS L3VPN Practice Question

This 300-410 practice question tests your understanding of mpls l3vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.

An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on the ingress interface of a PE router in an MPLS L3VPN. The router is receiving VPN traffic from a customer edge (CE) router. The engineer notices that some legitimate traffic is being dropped by uRPF. The engineer verifies that the CE router has a route back to the source address in its routing table. What is the most likely explanation?

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.

Question 1hardmultiple choice

Read the full MPLS explanation →

A
Asymmetric routing is causing the return path to use a different interface, violating the strict uRPF check.
Correct. uRPF strict mode requires that the return path uses the same interface; asymmetric routing causes legitimate traffic to be dropped.
B
The uRPF 'allow-default' option is not configured, so default routes are not considered.
Why wrong: Incorrect. The 'allow-default' option allows uRPF to consider default routes, but the issue here is asymmetric routing, not default routes.
C
The CE router is not advertising the source network to the PE via BGP.
Why wrong: Incorrect. If the CE were not advertising the source network, the PE would not have a route to the source, and uRPF would drop the packet. But the question states the CE has a route back.
D
The uRPF mode is set to 'loose' instead of 'strict', causing all traffic to be dropped.
Why wrong: Incorrect. Loose mode only checks that a route exists, not the interface; it would not cause drops due to asymmetric routing.

Full breakdown with real-world context →

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

✓

Asymmetric routing is causing the return path to use a different interface, violating the strict uRPF check.

uRPF strict mode checks that the source address of an incoming packet has a route in the routing table that points back to the same interface on which the packet was received. If there is asymmetric routing (i.e., the return path takes a different interface), uRPF strict mode will drop the packet. In an MPLS L3VPN, traffic from the CE to the PE may take one path, but return traffic from the PE to the CE may take a different path (e.g., due to load balancing or different routing policies). This is a common edge case. The solution is to use uRPF loose mode or to ensure symmetric routing.

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.

✓
Asymmetric routing is causing the return path to use a different interface, violating the strict uRPF check.
Why this is correct
Correct. uRPF strict mode requires that the return path uses the same interface; asymmetric routing causes legitimate traffic to be dropped.
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 uRPF 'allow-default' option is not configured, so default routes are not considered.
Why it's wrong here
Incorrect. The 'allow-default' option allows uRPF to consider default routes, but the issue here is asymmetric routing, not default routes.
✗
The CE router is not advertising the source network to the PE via BGP.
Why it's wrong here
Incorrect. If the CE were not advertising the source network, the PE would not have a route to the source, and uRPF would drop the packet. But the question states the CE has a route back.
✗
The uRPF mode is set to 'loose' instead of 'strict', causing all traffic to be dropped.
Why it's wrong here
Incorrect. Loose mode only checks that a route exists, not the interface; it would not cause drops due to asymmetric routing.

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 300-410 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 300-410 question test?

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: Asymmetric routing is causing the return path to use a different interface, violating the strict uRPF check. — uRPF strict mode checks that the source address of an incoming packet has a route in the routing table that points back to the same interface on which the packet was received. If there is asymmetric routing (i.e., the return path takes a different interface), uRPF strict mode will drop the packet. In an MPLS L3VPN, traffic from the CE to the PE may take one path, but return traffic from the PE to the CE may take a different path (e.g., due to load balancing or different routing policies). This is a common edge case. The solution is to use uRPF loose mode or to ensure symmetric routing.

What should I do if I get this 300-410 question wrong?

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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