Cisco CCNA Physical Interfaces, Cabling, and Interface Errors — Questions and Answers
Question 1: What is the maximum supported segment length for 1000BASE-T over Cat5e/Cat6 UTP copper?
- 55 meters
- 100 meters (Correct answer)
- 185 meters
- 550 meters
Correct answer: 100 meters
1000BASE-T, like 10BASE-T and 100BASE-TX, supports runs of up to 100 meters on twisted-pair copper. Longer distances need fiber.
Question 2: Which statement correctly compares single-mode fiber with multimode fiber?
- Single-mode has a larger core and is used for short runs
- Single-mode has a smaller core, usually uses lasers, and supports much longer distances (Correct answer)
- Multimode supports longer distances because it carries more light modes
- Both support the same distances; only the connector differs
Correct answer: Single-mode has a smaller core, usually uses lasers, and supports much longer distances
Single-mode fiber has a narrow core of about 9 µm and usually uses a laser light source. This reduces modal dispersion and allows runs of many kilometers. Multimode has a wider core (50/62.5 µm), uses cheaper LEDs or VCSELs, and is limited to shorter distances.
Question 3: 'show interfaces' on a switch port shows a growing count of late collisions. What is the most likely cause?
- A duplex mismatch (Correct answer)
- An incorrect VLAN assignment
- STP blocking the port
- A missing default gateway
Correct answer: A duplex mismatch
Late collisions are the classic sign of a duplex mismatch. The half-duplex side detects collisions after the first 64 bytes because the full-duplex side transmits whenever it wants. Cables longer than spec can also cause them, but duplex mismatch is the usual answer.
Question 4: An interface shows a steadily rising number of CRC errors but no collisions, and both ends are set to full duplex. What is the most likely cause?
- A routing loop
- A bad cable, a bad connector, or electromagnetic interference (Correct answer)
- An ARP table overflow
- Too many VLANs on the trunk
Correct answer: A bad cable, a bad connector, or electromagnetic interference
CRC errors mean the frames arrived corrupted. With duplex set correctly on both ends, the likely cause is the physical layer: a damaged cable or connector, EMI, or a cable longer than spec.
Question 5: What does Auto-MDIX do on a switch port?
- It automatically negotiates the port's VLAN
- It detects the cable type and adjusts transmit/receive pairs, so either straight-through or crossover cable works (Correct answer)
- It powers connected devices over Ethernet
- It aggregates multiple links into one
Correct answer: It detects the cable type and adjusts transmit/receive pairs, so either straight-through or crossover cable works
Auto-MDIX automatically swaps the transmit and receive pairs as needed. This removes the need to choose between straight-through and crossover cables when connecting similar or different devices.
Question 6: Switch A's 10/100 port is hard-coded to 100 Mbps full duplex with autonegotiation disabled. Switch B's port is set to auto. What will Switch B most likely choose?
- 100 Mbps, full duplex
- 100 Mbps, half duplex (Correct answer)
- 10 Mbps, half duplex
- The link will not come up
Correct answer: 100 Mbps, half duplex
Switch B can detect the 100 Mbps speed from the signal. Because no autonegotiation messages arrive, it falls back to half duplex at 10/100 Mbps. The result is a duplex mismatch: A is full duplex, B is half duplex.
