Cisco Certified Network Associate (CCNA) 200-301 Practice Test 4
Question 30
Two data-center rooms are 75 meters apart. Both switches have 10GBASE-SR multimode transceivers, and the installed OM4 fiber passes optical power testing. Which medium fits these installed optics?
- Substitute single-mode fiber on one side while leaving both 10GBASE-SR multimode optics unchanged.
- Use the OM4 multimode fiber plant with compatible 10GBASE-SR optics at both ends.
- Run a 75-meter analog telephone pair without Ethernet cable qualification because all twisted pairs carry 10 Gb/s.
- Use 1000BASE-LX optics at one end and 10GBASE-SR at the other because the wavelength difference is harmless.
Explanation
Correct Answer
B:Short-reach 10GBASE-SR optics are specified for multimode fiber with a defined optical link budget. A 75-meter OM4 span is normally within supported reach; installed optics and supported media must be matched instead of selecting fiber based only on distance.
Incorrect Answers
A:Changing media without matching the optical transceiver specification is not a valid interoperable design.
C:A telephone pair without the required Ethernet PHY/cable category is not the specified 10GBASE-SR fiber plant.
D:Different speeds and transceiver modes cannot establish the intended compatible 10-Gb/s link.
Question 31
Two enterprise routers are joined by one switched Ethernet point-to-point link operating full duplex, with no shared hub or coaxial bus. Which statement best describes normal link behavior?
- Every full-duplex Ethernet frame must first win a CSMA/CD collision backoff with the far-end router.
- The link behaves like a shared coaxial bus, so collisions between unrelated campus workstations are expected on this segment.
- Only one router may transmit in each second even though the negotiated link is full duplex.
- Full-duplex Ethernet sends and receives simultaneously without CSMA/CD collisions on this dedicated point-to-point segment.
Explanation
Correct Answer
D:A correctly negotiated full-duplex switched Ethernet link is not a shared collision domain. Transmit and receive paths operate concurrently, and classic half-duplex collision handling is not used. A link problem should be investigated with operational duplex and error counters, not assumed shared-media contention.
Incorrect Answers
A:Collision detection/backoff is a half-duplex shared-medium mechanism, not normal full-duplex operation.
B:The topology is dedicated point-to-point and has no shared coaxial collision medium.
C:Full duplex supports simultaneous transmit/receive, not manually serialized turns.
Question 32
A 120-meter copper access channel runs beside high-voltage machinery. Users report intermittent packet loss. `show interfaces` reports increasing CRC and input errors during motor operation; IP addressing and routes stay unchanged. Which investigation is most appropriate?
- Change the static default route solely because corrupted Layer 2 frames appear during motor startup.
- Disable TCP retransmission on hosts so the interface CRC counter no longer registers Ethernet frame errors.
- Inspect and certify the overlength UTP run and EMI exposure, then shorten/re-route or shield the cable as supported before changing IP policy.
- Convert the access switchport to VLAN 1 because a particular VLAN ID determines electromagnetic noise.
Explanation
Correct Answer
C:Twisted-pair Ethernet has a supported channel budget and is sensitive to installation quality and interference. A long noisy run with time-correlated CRC errors points to a physical integrity issue; CRC increments occur below IP route or ACL decisions.
Incorrect Answers
A:Routing changes cannot repair a noisy copper pair or overlength channel.
B:CRC is a link-layer frame integrity check; transport recovery does not prevent corruption.
D:VLAN labels do not control copper interference or channel length.
Question 33
Two switches have a 3-kilometer single-mode fiber plant. After a transceiver swap, the interfaces remain down/down. One end reports 1310-nm optics and the other 1550-nm optics with no compatible BiDi pair; routing was unchanged. Which diagnosis should be prioritized?
- The optical transceivers are an incompatible wavelength/pair combination; install a matched supported optic pair and verify received light levels.
- The OSPF area ID must differ because it directly controls the laser wavelength on each Ethernet port.
- The remote switch has an incorrect IPv4 default gateway, which by itself keeps optical carrier down.
- Change only the VLAN native tag while keeping an unmatched optic pair, because 802.1Q restores lost optical power.
Explanation
Correct Answer
A:A fiber interface cannot establish a usable optical signal when opposite ends use incompatible transmit/receive specifications. The wavelength labels and loss readings are direct Layer 1 evidence. The choice of routing protocols cannot repair an optical receive mismatch.
Incorrect Answers
B:OSPF hello matching operates above physical optical signal acquisition.
C:An IP route cannot change the optical PHY light budget or transceiver compatibility.
D:VLAN tagging follows successful frame reception; it does not create optical sync.
Question 34
A pair of switches uses 10GBASE-SR multimode optics over a 700-meter older OM2 fiber path. Both optics are healthy but the measured receiver level falls below the vendor-supported limit and frames error under load. The connectors fit. What design assumption is false?
- The Layer 3 IP default route sets the laser power budget for a 700-meter fiber span.
- A native VLAN mismatch alone explains the receiver optical level falling below minimum during link training.
- All multimode fiber lengths are supported at 10 Gb/s as long as connector geometry matches.
- Physical connector compatibility does not make the 700-meter OM2 channel fall within the supported 10GBASE-SR optical reach/budget.
Explanation
Correct Answer
D:Optical interfaces must meet fiber-grade and transceiver-specific distance and insertion-loss specifications. A nominally fitting connector says nothing about whether a long OM2 path is supported by short-range 10GBASE-SR, which typically has a much shorter qualified reach.
Incorrect Answers
A:Routing parameters do not define the optical power loss budget.
B:Native VLAN classification cannot reduce measured optical receive power.
C:Supported distance depends on the optic and fiber grade; this is the mistaken premise.
Question 35
Two switches have a direct copper link. Switch A is forced to 100 Mb/s full duplex while Switch B autonegotiates to 100 Mb/s half duplex. A reports FCS errors and B reports late collisions under heavy traffic. No hub is attached. Which root cause fits?
- Normal CSMA/CD contention from a shared repeater hub, although no shared hub exists.
- An OSPF reference-bandwidth difference causes Ethernet half-duplex detection on one PHY.
- A duplex mismatch from one side forced and the other negotiating; align both ends to compatible speed/duplex negotiation settings.
- A wrong DNS resolver causes the hardware to report late collisions under saturation.
Explanation
Correct Answer
C:When one end is forced full duplex and the other falls back to half duplex, the half-duplex device can see collisions while the full-duplex side sees corrupt frames. The link can remain physically up, so operational duplex values and counters matter more than assumptions about a nonexistent hub.
Incorrect Answers
A:A dedicated switched point-to-point link should not exhibit ordinary hub contention.
B:OSPF metrics cannot control physical duplex negotiation.
D:DNS failures affect application resolution, not Ethernet collision counters.
Question 36
Technicians replace a damaged 75-meter copper cable between a switch and router. Before the repair, the link displayed increasing CRC errors and negotiated 100 Mb/s despite 1-Gb/s capable ports. What postrepair evidence best proves the physical fault is corrected?
- The running-config still contains the original static IPv4 default route, regardless of current speed or errors.
- The DNS cache has the same A record before and after the repair, without checking Ethernet counters.
- The switch inventory lists the expected model, even though the port still reports 100 Mb/s and growing CRC counts.
- `show interfaces` and interface status now report the supported 1-Gb/s operational speed/duplex, a stable link and no increasing CRC/input error counters under load.
Explanation
Correct Answer
D:A physical-layer acceptance check verifies the actual negotiated mode and error deltas while traffic runs. A one-time up status is insufficient if corrupted frames continue. The speed/CRC evidence directly addresses the reported cable fault.
Incorrect Answers
A:Preserving a route does not prove that the repaired physical cable meets operational requirements.
B:DNS record consistency cannot establish physical frame integrity.
C:Device identity is not the acceptance criterion and the counters show ongoing link trouble.
Question 37
A 2-kilometer single-mode fiber link repeatedly loses signal after replacing one optic. The hardware inventory says the two installed transceivers support different wavelength pairings. Which verification best tests the optical root cause?
- Compare only the routers’ OSPF reference bandwidths because they determine the optical laser modulation rate.
- Confirm that a local host has DNS configured; that proves opposite optical transmit frequencies are matched.
- Compare both optic part numbers, supported wavelengths and reach specifications, then inspect transmit and receive optical power on each interface.
- Check that VLAN 20 exists in the local switch database and skip the optical power readings, even though signal drops.
Explanation
Correct Answer
C:Matched optics must interoperate with the fiber plant in direction, wavelength, power budget and reach. Digital optical monitoring readings can identify missing or weak received light, while inventory confirms module compatibility. Neither an IP route nor VLAN state can substitute for optical measurement.
Incorrect Answers
A:An OSPF routing cost reference does not govern physical transceiver wavelength.
B:DNS has no bearing on physical optical signal compatibility.
D:VLAN membership does not establish transceiver interoperability or received light margin.
Question 38
Two directly connected Gigabit Ethernet ports intermittently fail autonegotiation after a technician forces duplex on one side but leaves the peer auto. Which action best restores a supported full-duplex link without relying on a shared-media collision mechanism?
- Enable CSMA/CD backoff at both ends because a full-duplex point-to-point Gigabit link must share a collision domain.
- Change the VLAN native ID to 1 because it directly renegotiates physical duplex regardless of PHY settings.
- Configure a longer IPv4 subnet mask so the switch PHY learns full duplex from the network prefix.
- Restore compatible auto speed/duplex negotiation on both Gigabit Ethernet ends and confirm their operational mode and clean interface counters.
Explanation
Correct Answer
D:For normal 1000BASE-T Ethernet, supported autonegotiation is important for link parameters including master/slave resolution, and both sides should agree on the operational mode. Forcing one side while leaving the other automatic can produce negotiation failures or mismatched behavior; verification uses both endpoints.
Incorrect Answers
A:Full duplex avoids classic shared-medium collision operation.
B:802.1Q VLAN tagging is independent of physical duplex negotiation.
C:Subnetting cannot affect the link transceiver negotiation state.
Question 39
A switch's Gi1/0/17 receives 4,600 frames with CRC errors over ten minutes, although its IP default route and upstream routing table are unchanged. After replacing the patch cable, which action best isolates the remaining fault before any static-route changes?
- Delete the default route and configure a new next-hop IP on the router.
- Change the access interface from VLAN 17 to VLAN 1 without checking link counters.
- Inspect `show interfaces gi1/0/17` for new CRC/input-error increments, test the cable and optics, and compare speed/duplex on both link ends.
- Enable a DHCP relay helper on Gi1/0/17 to forward all client broadcasts.
Explanation
Correct Answer
C:CRC counters reflect corruption detected while receiving Ethernet frames. A static IP route does not correct electrical interference, failing cable pairs, marginal optics, or inconsistent duplex behavior on the link. Check the new error-count delta under traffic, replace or test physical media, and confirm the adjacent device agrees on negotiated speed and duplex. If the link is physically sound and the errors stop after replacement, verify Layer 3 reachability separately. This sequence tests the actual reported symptom instead of changing an unrelated routing decision.
Incorrect Answers
A:A default route determines forwarding when no more specific destination prefix matches. It does not govern the integrity of incoming Ethernet frames or explain a rising physical-interface CRC counter. Removing it can break legitimate off-subnet traffic while leaving corrupted frames unchanged. Inspecting physical error counters and negotiation is the relevant first step here.
B:Changing the access VLAN might move the endpoint into another broadcast domain, but it cannot repair a frame's failed Ethernet CRC check. The observed error is on the receiving physical interface, not a confirmed VLAN membership mismatch. A VLAN change could also strand the host in the wrong IP subnet. Verify physical error counts before touching segmentation.
D:DHCP relay forwards selected client broadcasts between subnets. It neither repairs malformed Ethernet frames nor makes an incompatible cable or duplex setting valid. Moreover a host-facing Layer 2 port may not be the correct routed interface for an IP helper. The diagnostic clue is new CRC errors, which require a physical-link investigation.
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.
