N10-009 - Network Troubleshooting (24% of the exam) - Section 5.4

Troubleshoot common wireless connectivity and general networking issues, including DHCP, DNS, addressing, interference, and routing problems.

Troubleshoot general service failures - DHCP exhaustion or a rogue server, DNS misconfiguration, duplicate or APIPA addressing, incorrect default gateway or subnet mask, and routing-loop or asymmetric-path symptoms. Diagnose wireless-specific faults: channel overlap and interference, insufficient signal strength or coverage, band steering, and authentication or encryption mismatches between client and access point.

DHCP/DNS issuesAPIPA and addressing errorsWireless interferenceSignal strength and coverageDefault gateway misconfiguration

Practice question for this objective

Free sampleNetwork Troubleshootinghard

A technician adds a new VLAN for a branch office and connects its clients through a layer 3 switch that routes to a central DHCP server on a different subnet. Clients on established VLANs still lease addresses normally, but every client on the new VLAN fails to obtain one even though the central server has thousands of free addresses. What should the technician configure first?

AA larger DHCP scope on the central server so that it can accommodate the new VLAN's additional clients.
BA static DNS entry for the DHCP server so that new clients can locate the server by its name.
CA shorter lease time on the central server so that leased addresses are recycled more quickly.
DAn IP helper address on the new VLAN's gateway interface so that DHCP broadcasts are relayed to the central server. Correct

A routed subnet with no DHCP relay cannot reach a DHCP server on another subnet because routers do not forward DHCP broadcasts. DHCP clients begin by broadcasting a DISCOVER, and broadcasts stay within their own subnet because routers do not forward them; an IP helper address on the client subnet's gateway rewrites those broadcasts as unicast packets aimed at the central DHCP server, which is why only the new, un-relayed VLAN fails while relayed VLANs succeed.

Why A is wrong: The server already has thousands of free addresses, so scope size is not the constraint; a larger scope changes nothing when the requests never arrive.

Why B is wrong: Clients locate a DHCP server by broadcast, not by name, so a DNS entry for the server is irrelevant to obtaining a lease.

Why C is wrong: Shorter leases affect only how quickly addresses are reused and do nothing to carry broadcasts from a subnet that cannot reach the server at all.

Why D is correct: DHCP DISCOVER messages are broadcasts that routers do not forward, so a new routed subnet needs an IP helper on its gateway to unicast those requests to the central server; without it, clients on that VLAN never reach the server.

See more N10-009 practice questions, answers explained.

Exam traps in Network Troubleshooting

Answers that look right on this material and are not. Each one is a distractor from a different question in the N10-009 bank for this domain.

The authorised DHCP server's scope has run out of available addresses for the affected clients.
Why it is wrong: Scope exhaustion would leave some clients with no address at all, not with a working address that points at a different, incorrect gateway.
Increase the transmit power of the existing access point to its maximum setting.
Why it is wrong: Raising downlink power can extend the cell, but the laptop's own transmit power is unchanged, so the weak uplink and low RSSI persist and interference elsewhere may rise.
Coverage is insufficient, so raising transmit power on the two access points will resolve it.
Why it is wrong: A healthy -55 dBm everywhere shows coverage is already strong, so more power adds no capacity and only increases interference between the two access points.

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