Network Troubleshooting

486 soru

Soru 481Soru

A network technician is troubleshooting an issue where a dual-stack client workstation fails to connect to an internal host named `app-server.corp.local` over IPv6. The technician runs `nslookup -type=AAAA app-server.corp.local` and receives a response stating that the domain name does not exist. However, executing `nslookup app-server.corp.local` successfully returns the IPv4 address 10.1.10.4510.1.10.45. Which of the following is the most likely cause of this issue?

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Cevap: The DNS server has a configured IPv4 (A) record for the target host but lacks a corresponding IPv6 (AAAA) record.

Cevap

The DNS server has a configured IPv4 (A) record for the target host but lacks a corresponding IPv6 (AAAA) record.
The standard `nslookup` command requests IPv4 address (A) records by default, which resolved successfully to 10.1.10.4510.1.10.45. Specifying `-type=AAAA` forces the utility to explicitly query for an IPv6 host record. The non-existent domain error returned by the server confirms that while the host exists with an IPv4 A record, no corresponding IPv6 AAAA record has been added to DNS.

Adım Adım Çözüm

1
Analyze the default `nslookup` command execution.
Executing `nslookup app-server.corp.local` returns 10.1.10.4510.1.10.45, confirming active connectivity to DNS and the presence of an IPv4 (A) record.
By default, `nslookup` queries for type A records when no explicit type parameter is specified.
2
Analyze the targeted `-type=AAAA` command execution.
The query `nslookup -type=AAAA app-server.corp.local` returns a non-existent domain/record failure.
The `-type=AAAA` switch explicitly restricts the DNS query to IPv6 host records.
3
Synthesize the outputs to determine the missing configuration.
Because IPv4 address resolution succeeds but IPv6 address resolution fails, the DNS zone file contains an A record but lacks an AAAA record.
Dual-stack network operation requires separate A (IPv4) and AAAA (IPv6) resource records for host resolution.

Anahtar Kavram

DNS Command-Line Utilities and Resource Record Query Analysis
Soru 482Soru

Following an office wall installation project, a workstation loses network access. A network technician suspects that a 120-meter Category 6 unshielded twisted-pair (UTP) cable running through the wall cavity has been physically severed by a drywall fastener. Which diagnostic tool should the technician use to determine the exact distance along the cable run to the location of the break?

Cevabı ve açıklamayı göster

Cevap: Time-Domain Reflectometer (TDR)

Cevap

Time-Domain Reflectometer (TDR)
A Time-Domain Reflectometer (TDR) functions by emitting high-frequency electrical pulses down copper cable conductors. When the pulse strikes an impedance anomaly—such as an open circuit caused by a severed wire—a portion of the signal reflects back to the instrument. By measuring the round-trip time of the reflection, the TDR accurately determines the distance to the fault.

Adım Adım Çözüm

1
Analyze the physical failure scenario and requirement
The scenario identifies a physical break (open fault) inside a copper UTP cable run behind walls where the distance to the break must be accurately pinpointed.
Measuring distance to the break allows technicians to access and repair damaged cabling with minimal structural disruption.
2
Evaluate copper testing tool capabilities for fault location
A Time-Domain Reflectometer (TDR) sends electrical pulses down the copper wire and measures the return reflection caused by the open circuit to calculate exact distance.
TDR technology utilizes signal reflection and Nominal Velocity of Propagation (NVP) to measure distance to impedance changes.

Anahtar Kavram

Selecting Physical Cabling Diagnostic Tools
Soru 483Soru

A desktop technician is systematically troubleshooting a workstation on a newly configured subnet (10.80.12.0/2410.80.12.0/24) that cannot access network resources. The workstation failed to acquire a lease from the enterprise DHCP server located on a separate server subnet (10.10.2.0/2410.10.2.0/24). Following standard troubleshooting methodology from the local client to the centralized infrastructure, in what order should the technician perform these diagnostic and verification steps?

Öğeleri doğru sıraya koymak için sürükleyin

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Cevap

The correct logical sequence begins with checking the local host IP configuration via `ipconfig /all`, followed by verifying switch port VLAN assignment, then checking the `ip helper-address` configuration on the gateway router, and finally auditing the centralized DHCP server scope for available addresses.
The standard troubleshooting methodology moves systematically from the affected endpoint up to the remote infrastructure. First, verifying the local host configuration with `ipconfig /all` identifies symptoms such as APIPA assignment. Next, checking the physical/datalink switchport ensures the client is in the proper VLAN segment. Third, checking the gateway's IP helper address verifies that DHCP broadcast requests are properly relayed across subnets to the server's IP address. Finally, auditing the central DHCP server scope verifies server-side availability and pool capacity.

Adım Adım Çözüm

1
Inspect local host configuration.
Confirm whether the client has an APIPA address (169.254.x.x169.254.x.x) or proper link-local connectivity.
Always start troubleshooting at the affected host before investigating network infrastructure.
2
Verify local switchport layer 2 configuration.
Ensure the access port is mapped to the intended VLAN.
If the switch port is in an incorrect or unrouted VLAN, DHCP Discover broadcasts will not reach the gateway.
3
Check gateway DHCP relay agent settings.
Verify that `ip helper-address 10.10.2.x` is present on the SVI/interface.
DHCP broadcasts cannot cross routers unless an IP helper address forwards them to the remote DHCP server.
4
Check remote DHCP server status and scope.
Ensure the scope for 10.80.12.0/2410.80.12.0/24 is active and has unallocated IP addresses available.
Scope exhaustion or disabled scopes prevent new leases even when network relay path is functioning properly.

Anahtar Kavram

Methodological IP and DHCP Service Troubleshooting
Tahmini Süre:1m 30s
Soru 484Soru

A network administrator receives alerts that newly deployed IP security cameras in a logistics facility are unable to communicate with the central surveillance management server. Examination of a affected camera's network configuration reveals an IP address of 192.168.1.105192.168.1.105 with a default gateway of 192.168.1.1192.168.1.1. However, the camera VLAN is defined as 10.120.45.0/2410.120.45.0/24 with leases managed by a central enterprise DHCP server at 10.100.10.5010.100.10.50. Which of the following is the most likely cause of this issue?

Cevabı ve açıklamayı göster

Cevap: A rogue DHCP server is connected to the camera VLAN and responding to lease requests faster than the authorized DHCP server.

Cevap

The presence of a rogue DHCP server on the local subnet responding to client requests.
When network clients receive valid IP leases belonging to an incorrect subnet rather than APIPA addresses or no IP at all, an unauthorized (rogue) DHCP server attached to the local switchport segment is responding to DHCPDISCOVER broadcast packets before the legitimate remote or local DHCP server.

Adım Adım Çözüm

1
Analyze the reported IP addressing symptoms
The client received an IP address (192.168.1.105192.168.1.105) and gateway (192.168.1.1192.168.1.1) belonging to a completely different subnet than the designated VLAN subnet (10.120.45.0/2410.120.45.0/24).
Determining whether an address is APIPA, misconfigured statically, or leased from an unexpected range helps narrow down DHCP infrastructure issues.
2
Evaluate potential failure modes
Because the host received a valid (non-APIPA) DHCP lease, a DHCP server answered the DHCPDISCOVER request. Since the authorized server only distributes 10.120.45.0/2410.120.45.0/24 addresses, an unauthorized server on the local broadcast domain must be answering.
DHCP discovery uses local broadcasts; the fastest DHCP OFFER response is accepted by the client host.
3
Select the correct root cause
A rogue DHCP server on the broadcast domain is intercepting and responding to client discovery messages.
Rogue DHCP servers (e.g., misconfigured consumer routers connected to switchports) respond with their own local pool settings.

Anahtar Kavram

Rogue DHCP Server Identification and Symptoms
Tahmini Süre:1m 30s
Soru 485Soru

A network engineer is deploying new point-of-sale (POS) terminals on VLAN 105 (192.168.105.0/24192.168.105.0/24). The enterprise central DHCP server resides on VLAN 10 (10.10.10.210.10.10.2). During initial testing, the POS terminals fail to obtain leased IP addresses and continuously self-assign IP addresses in the range of 169.254.1.1169.254.1.1 through 169.254.254.254169.254.254.254. Which of the following potential root causes or diagnostic actions should the engineer investigate to resolve this connectivity issue? (Select TWO.)

Geçerli olan tümünü seçin

Cevabı ve açıklamayı göster

Cevap: Verify whether an IP helper address pointing to 10.10.10.210.10.10.2 is configured on the Layer 3 interface serving VLAN 105.; Inspect the DHCP server scope for 192.168.105.0/24192.168.105.0/24 to verify if the address pool has reached full exhaustion.

Cevap

The network engineer should verify that a DHCP relay agent (IP helper address pointing to 10.10.10.2) is active on the VLAN 105 gateway interface and inspect the DHCP server scope to ensure available lease pool capacity.
When hosts receive APIPA addresses (169.254.x.x), it indicates that DHCP lease negotiation failed. For clients on VLAN 105 communicating with a DHCP server on VLAN 10, a DHCP relay agent (IP helper address) is required on the router interface to relay Layer 2 broadcasts across subnets as unicast traffic. Additionally, if the DHCP scope on the server is exhausted, no leases can be issued, resulting in the same APIPA fallback behavior.

Adım Adım Çözüm

1
Analyze the client symptom
The terminals display APIPA addresses (169.254.x.x), indicating that DHCP Discover broadcast requests are failing to receive a DHCP Offer.
When a host does not receive a response from a DHCP server, automatic private IP addressing (APIPA) assigns a non-routable link-local address.
2
Examine cross-subnet broadcast forwarding requirements
Broadcast packets for DHCP Discover cannot cross router boundaries without explicit relay configuration.
Configuring an IP helper address on the VLAN 105 Layer 3 interface converts DHCP broadcast requests into unicast packets routed to 10.10.10.2.
3
Check DHCP server scope allocation status
Confirming scope availability ensures valid IP leases are present for incoming requests.
Exhaustion of available IP addresses within a DHCP pool prevents new clients from acquiring dynamic leases even if broadcast forwarding is functional.

Anahtar Kavram

Cross-subnet DHCP Relay and APIPA Troubleshooting
Soru 486Soru

Match each physical layer cabling fault or task scenario with the diagnostic tool or equipment best suited to identify or resolve it.

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Öğeler

A Category 6 UTP cable installation fails performance testing due to pairs 3 and 6 being split across different physical twisted pairs while maintaining pin-to-pin electrical continuity.
An 850 nm multimode fiber optic link exhibits high optical insertion loss caused by microscopic dust and oil contamination on the transceiver ferrule endface.
A metallic twisted-pair cable run experiences a complete conductor break inside a wall conduit, requiring the technician to measure the exact distance to the fault.
A network technician needs to trace an unlabelled horizontal Ethernet cable from an office wall jack back to its corresponding port on a dense patch panel.

Eşleşmeler

Cevabı ve açıklamayı göster

Cevap

1. Split pair condition -> Cable Certifier with advanced Wiremap analysis; 2. Contaminated fiber ferrule -> Fiber Optical Inspection Scope and Cleaning Kit; 3. Conductor break in conduit -> Metallic Time-Domain Reflectometer (TDR); 4. Unlabelled cable tracing -> Tone Generator and Inductive Probe.
Each physical layer diagnostic tool is designed for specific cable testing tasks: Cable Certifiers detect high-frequency cabling parameters and split pair errors; Fiber Inspection Scopes inspect ferrule endfaces for contamination; TDR devices measure time domain reflections to find exact distance to cable breaks; and Toner Probes trace unlabelled wires across patch panels.

Adım Adım Çözüm

1
Analyze the split pair scenario
Identify that pin-to-pin continuity is present but pair twisting is violated, which requires advanced wiremap capability provided by a cable certifier.
Basic continuity testers cannot detect split pairs because electrical continuity exists across individual conductors.
2
Analyze optical attenuation caused by contamination
Match with Fiber Optical Inspection Scope and Cleaning Kit.
Direct visual inspection of the fiber ferrule endface is required to confirm contamination before applying proper cleaning techniques.
3
Analyze physical conductor break location task
Match with Metallic Time-Domain Reflectometer (TDR).
TDR measures signal reflections to determine the exact distance along the cable to impedance anomalies such as opens or breaks.
4
Analyze unlabelled cable tracing task
Match with Tone Generator and Inductive Probe.
A tone generator places an audible signal onto a cable wire, allowing non-contact identification at the patch panel using the inductive probe.

Anahtar Kavram

Selecting appropriate diagnostic equipment for copper and fiber physical layer troubleshooting.
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