Cisco Certified Network Associate (CCNA) 200-301 Practice Test 5
Question 40
A switch uplink was upgraded, but its installed transceiver supports only 1 Gb/s while the far-end port is forced to 10 Gb/s. Both ends are administratively enabled. Which change is most likely to establish a functioning physical link?
- Force both ports to full duplex without checking the transceiver speed rating.
- Add a static route to the remote management subnet on both switches.
- Use compatible optics and interfaces at a mutually supported link speed, then confirm link negotiation.
- Increase the switchport MTU to 9000 bytes on the 1 Gb/s interface.
Explanation
Correct Answer
C:Both ends of an Ethernet link must support the selected media and physical bit rate; a 1 Gb/s-only module cannot satisfy a 10 Gb/s-only setting. Match the actual SFP/NIC and switchport capabilities, then verify operational speed and link state rather than merely administrative state.
Incorrect Answers
A:Duplex agreement matters after a link can negotiate and establish a supported physical signaling rate. Merely forcing full duplex cannot make a 1 Gb/s-only optic transmit a 10 Gb/s signal; unsupported speed is the immediate blocker.
B:A routed management path cannot change the Ethernet PHY and transceiver capabilities. The port must become operationally up at a common physical speed before any VLAN or IP forwarding diagnosis is meaningful.
D:MTU sets the largest accepted layer-3 payload/frame size, not the optical/electrical link symbol rate. Jumbo-frame configuration will not compensate for an unsupported transceiver speed.
Question 41
A half-duplex port connects to a full-duplex peer. The half-duplex side reports late collisions, and the full-duplex side records increasing FCS errors under load. Which diagnosis best explains the asymmetric counters?
- An OSPF authentication mismatch prevents an IP routing adjacency.
- A duplex mismatch: one end uses CSMA/CD collision behavior while the other transmits as full duplex, producing loss and errors during simultaneous traffic.
- An 802.1Q trunk excludes the voice VLAN but permits the data VLAN.
- An ACL implicit deny blocks TCP sessions without producing Ethernet errors.
Explanation
Correct Answer
B:On a half-duplex Ethernet interface, simultaneous transmissions can be treated as collisions; the full-duplex peer does not expect collision arbitration. This mismatch can lead to late-collision indications on one side and FCS/CRC symptoms on the other, especially under traffic. Compare actual operating settings from both interfaces rather than only configured intentions. Align supported speed and duplex, preferably compatible negotiation on both sides, then clear or baseline counters and confirm the collision and error rates stop increasing under a repeatable traffic test.
Incorrect Answers
A:OSPF authentication parameters are checked at the routing-protocol level, after the underlying Layer 2 link can carry frames. Such a mismatch normally prevents an adjacency or route learning, not collision counters from rising on one Ethernet endpoint. The half/full duplex evidence identifies a physical link problem before OSPF should be considered.
C:A missing allowed VLAN on a trunk can selectively disrupt traffic for that VLAN while other VLANs work. It does not ordinarily explain why one port sees late collisions and its peer sees FCS errors. Those counters are associated with Ethernet frame transmission and reception, not whether a valid 802.1Q tag is admitted.
D:An ACL can deny IP packets according to source, destination, and protocol fields, but a denied packet does not need to have a bad Ethernet frame check sequence. The reported symptoms arise beneath the IP filtering layer. Verify interface duplex state before attributing the issue to a Layer 3 access list.
Question 42
A switchport receives frames with steadily increasing input CRC/FCS errors; a changed patch cable alters the error rate. An engineer suspects an IP ACL is dropping the packets. Which conclusion best fits the observed counters?
- The ACL is certainly corrupting each Ethernet FCS after successful frame reception.
- The switch is receiving corrupted Ethernet frames; inspect cabling, PHY, transceiver and duplex before attributing the CRC increments to an IP ACL.
- The failing component must be an IPv4 static route because frame CRC checks are performed by routers only.
- A full MAC table is proof that all incoming frames pass CRC validation.
Explanation
Correct Answer
B:An Ethernet FCS failure is detected on received frames at Layer 2, potentially before IP packet processing. ACL denials are forwarding-policy decisions that do not normally increment physical CRC counters. Cable substitution affecting the error rate strengthens a link-quality hypothesis.
Incorrect Answers
A:Layer 3 ACL matching does not rewrite frame FCS to create receive CRC errors; the observed counter belongs to frame reception.
C:Ethernet switch interfaces check frame integrity independently of the IP routing table; a bad route does not produce input FCS corruption.
D:The MAC table reflects learned addresses, not a blanket guarantee of receive-frame integrity; CRC counters can rise despite learned neighbors.
Question 43
After an optic replacement, an Ethernet uplink reports administratively up but line protocol down. The local port is forced to 10 Gb/s; the installed far-end optic and port support only 1 Gb/s. Which diagnosis best explains the operational failure?
- A duplex mismatch on an otherwise operating link, usually investigated through collision and FCS counters.
- An IP subnet-mask mismatch preventing routing even though the Ethernet line protocol remains up.
- The two ends have incompatible physical speed/media capabilities, so the link cannot establish.
- An STP-blocked switchport that must be forwarding before its physical carrier comes up.
Explanation
Correct Answer
C:The local port is configured for a speed that the far end cannot physically support. Administrative up means it is enabled, not that the PHY negotiated or detected a compatible signal; match media, optics and rate before checking higher layers.
Incorrect Answers
A:Duplex mismatch can damage performance and produce error counters after Ethernet link establishment. The stated 10 Gb/s versus 1 Gb/s transceiver capability conflict explains the absence of operational carrier, not just degraded traffic.
B:Different IPv4 masks can prevent layer-3 reachability, but they do not normally cause an Ethernet port line protocol to be down when the electrical or optical link is healthy.
D:STP discards user frames on a nonforwarding logical port but does not normally remove the Ethernet PHY carrier itself. A down line protocol after an incompatible optic change points to link capability first.
Question 44
An access link was changed from a half/full duplex mismatch to matched duplex at both ends. Earlier, the half-duplex end counted late collisions and the full-duplex end counted FCS errors. Which post-change evidence most directly demonstrates the duplex fault is cleared?
- Both interfaces are administratively enabled, without inspecting negotiated duplex or data-plane counters.
- With traffic flowing, both peers report matching duplex and the late-collision and FCS error counters stop increasing.
- The IP route to the remote LAN remains in the routing table while no port counters are checked.
- Disable the port to keep FCS error counts from rising.
Explanation
Correct Answer
B:An asymmetric duplex mismatch commonly generates collision symptoms at the half-duplex endpoint and frame-check-sequence errors at its full-duplex peer. Matching duplex plus stable errors under load directly tests the original failure, not just whether the interface is up.
Incorrect Answers
A:Administrative up is only configuration permission. It does not prove matching duplex or the absence of late collisions and corrupt receive frames.
C:A layer-3 route can be present through a degraded duplex link. Routing-table status does not establish Ethernet frame integrity.
D:A shutdown port is not forwarding user traffic; zero error increments in that state are not evidence that the link has been repaired.
Question 45
After a cable with damaged conductors is replaced, the previously affected switchport comes up. The input CRC count had increased while hosts transmitted traffic. What result demonstrates that the physical-frame problem is no longer present?
- `show interfaces` shows the port up while received frames continue and the CRC/input-error counters remain stable over a measured interval.
- The interface is administratively disabled, so no CRC errors are counted.
- The router has a new static IPv4 route, while input CRCs still rise.
- The switch MAC table lists the peer MAC once, without checking continuing frame errors.
Explanation
Correct Answer
A:CRC/input-error counters detect frames that do not pass physical frame-integrity checks. A stable counter while traffic is actually received supports a repaired cable or PHY fault; compare before/after deltas, not absolute historical counts.
Incorrect Answers
B:No traffic arrives on a disabled interface; it cannot be used as a successful physical-layer acceptance test.
C:Routing information does not repair corrupted Ethernet frames arriving at a switchport.
D:A learned source MAC does not prove that subsequent frames avoid FCS corruption; inspect counter trends under normal load.
Question 46
Technicians replaced incompatible uplink optics with a supported pair and corrected speed settings. Which IOS evidence most directly verifies that the original physical-link failure has been resolved?
- A drop in CRC counters alone, without checking whether the interface is operationally connected.
- A successful management ping through a different backup uplink.
- `show interfaces status` reports the intended port as connected at the mutually supported operational speed, corroborated by interface line protocol up.
- The configuration says `no shutdown` on both ports, although operational status is not checked.
Explanation
Correct Answer
C:The port must show actual connected/up state and a speed supported by both installed modules. Operational evidence from `show interfaces status` and `show interfaces` directly validates the previously down link and the speed mismatch correction.
Incorrect Answers
A:CRC trends are useful for frame integrity on active links, but a nonoperational port may show no new CRCs simply because it receives no traffic. Carrier and negotiated speed must be checked for this failure.
B:A backup path can deliver the ping despite the repaired uplink still being down. A route-dependent reachability test alone cannot establish whether the specified physical port recovered.
D:Administrative enablement is necessary but not sufficient. `no shutdown` can coexist with line protocol down due to unsupported optic, loss of carrier or mismatched physical signaling.
Question 47
A client must transfer a file to a service listening on TCP port 443, preserving application byte order and recovering lost segments without application-specific resend code. Which transport behavior satisfies these needs?
- Use raw UDP datagrams and rely on their arrival order, without any reliability protocol in the application.
- Use Ethernet FCS checks as the only mechanism for end-to-end resend.
- Use TCP, which establishes a connection, numbers bytes, acknowledges delivered data and retransmits missing segments.
- Use ICMP echo requests with a TCP destination port to establish application delivery.
Explanation
Correct Answer
C:TCP provides an ordered reliable byte stream with sequence numbers, acknowledgments, retransmission and congestion control. A 443 server can accept a connection only if its TCP transport exchange succeeds; using UDP does not supply those guarantees automatically.
Incorrect Answers
A:UDP carries independent datagrams and has neither a built-in ordered byte stream nor automatic lost-datagram retransmission.
B:FCS rejects corrupted local frames but cannot guarantee recovery or ordering across an end-to-end routed path.
D:ICMP echo has neither TCP port numbers nor TCP connection state or byte-stream sequence recovery.
Question 48
A live sensor sends frequent timestamped telemetry updates where a delayed sample is less useful than a fresh one. The application can tolerate occasional loss and does not require in-order retransmission. Which transport design best fits?
- Use TCP to force recovery of every missing byte before delivering a later update, regardless of delay.
- Send timestamped updates using UDP and let the application decide whether to discard missing or stale samples.
- Use ARP as the transport for timestamped sensor samples across multiple IP networks.
- Choose TCP solely because it uses a three-way handshake, even though the application requires low-latency lossy delivery.
Explanation
Correct Answer
B:UDP has low transport overhead, no session establishment and no built-in retransmission, so the application can prioritize recency. It must still handle loss, sequence or freshness itself if those concerns matter.
Incorrect Answers
A:TCP reliability can add head-of-line blocking when a lost segment must be recovered, which conflicts with the stated priority for current data.
C:ARP resolves neighboring IP addresses to link-layer addresses; it is not an end-to-end telemetry transport.
D:A handshake does not make TCP a better fit for a stream explicitly designed to discard obsolete samples rather than recover them.
Question 49
A TCP file transfer loses a segment carrying bytes 2001–3000. Later segments arrive, and the receiver continues acknowledging the last contiguous byte stream. What mechanism lets the original transfer recover without application retry?
- The receiver silently treats the missing bytes as delivered because later TCP packets arrived.
- The sender changes transport to UDP midconnection and assumes UDP will retransmit the missing data.
- TCP uses cumulative acknowledgments and retransmission of the missing sequence range, restoring the ordered byte stream.
- Ethernet broadcasts the missing IP packet to every VLAN until the receiver responds.
Explanation
Correct Answer
C:The receiver cannot advance cumulative acknowledgment beyond the missing range until the gap is filled. TCP can retransmit after duplicate ACK signals or a retransmission timer, and in-order delivery resumes once the missing bytes arrive.
Incorrect Answers
A:TCP sequence numbers expose the gap; it cannot safely advance the contiguous byte stream across unknown missing content.
B:UDP has no TCP connection state, acknowledgments or retransmission semantics; switching protocols would not complete the existing file transfer.
D:Switch flooding is not an end-to-end TCP recovery mechanism and does not cross VLANs to retransmit selected TCP sequence numbers.
Question 50
A client sends repeated TCP SYN packets to a server on port 443 but never receives a SYN-ACK; a separate UDP DNS query still receives a reply. Which inference is most directly supported?
- UDP DNS succeeded, so a three-way TCP handshake must already have completed on port 443.
- A TCP SYN-ACK was received and acknowledged, proving the connection is already established.
- The TCP connection handshake is not completing on the tested destination/port; check listener, filtering and reverse reachability before blaming UDP.
- The sender is experiencing normal TCP data retransmissions after a completed connection.
Explanation
Correct Answer
C:TCP normally begins with SYN, SYN-ACK and ACK. Repeated unanswered SYNs establish that no usable SYN-ACK reached this client; they do not identify exactly which firewall, service or return-path issue blocked it. The separate UDP response does not validate TCP port 443.
Incorrect Answers
A:A UDP DNS exchange does not use TCP connection setup, and another service can work while a particular TCP port is filtered.
B:That contradicts the capture: no SYN-ACK appears, so the three-way handshake has not completed.
D:SYN retransmissions occur while attempting to establish the session, not after file-transfer payload has been acknowledged.
