Question 716 of 2,015

Infrastructure Security →mediumDrag & DropObjective-mapped

Quick Answer

The correct order for the uRPF check process steps is: receive packet, look up source IP in routing table, verify incoming interface matches best reverse path, then forward the packet if it matches or drop it if there is a mismatch. This sequence is rooted in the fundamental principle of unicast reverse path forwarding, which ensures that a packet’s source address is reachable via the same interface on which it arrived, thereby preventing IP spoofing attacks. On the ENCOR 350-401 exam, this drag-and-drop question tests your understanding of how uRPF operates in strict mode, where the router drops the packet if the incoming interface does not match the best reverse path in the routing table. A common trap is confusing the order of the lookup and the interface verification—remember that the source IP lookup must happen before the interface check. To lock in the steps, use the mnemonic “R-L-V-F-D”: Receive, Lookup, Verify, Forward or Drop.

350-401 Infrastructure Security Practice Question

This 350-401 practice question tests your understanding of infrastructure security. 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.

Drag and drop the steps of Unicast Reverse Path Forwarding (uRPF) check process into the correct order, from first to last.

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

Question 1mediumdrag order

Read the full NAT/PAT explanation →

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

✓

Router receives packet on an interface

uRPF first receives a packet on an interface, then looks up the source IP in the routing table, verifies that the incoming interface matches the best reverse path, and if it matches, forwards the packet; otherwise, it drops the packet.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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 350-401 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 350-401 question test?

Infrastructure Security — This question tests Infrastructure Security — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: Router receives packet on an interface — uRPF first receives a packet on an interface, then looks up the source IP in the routing table, verifies that the incoming interface matches the best reverse path, and if it matches, forwards the packet; otherwise, it drops the packet.

What should I do if I get this 350-401 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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Same concept, more angles

1 more ways this is tested on 350-401

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Drag and drop the steps of Unicast Reverse Path Forwarding (uRPF) check process into the correct order, from first to last.

medium

✓ A.Enable uRPF on the interface (strict or loose)
✓ B.Router receives packet on the interface
✓ C.Perform FIB lookup on source IP address
✓ D.Check if best reverse path uses same ingress interface
✓ E.Forward packet if valid or drop if spoofed

Why A: uRPF checks the source IP of incoming packets by first looking up the source in the FIB. If the best reverse path to the source uses the same interface, the packet is forwarded; otherwise, it is dropped. Strict mode requires exact match, while loose mode only requires a route.

Last reviewed: Jun 18, 2026

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