220-1101 Question 126
Single answerSwitchesA technician is replacing an old unmanaged 24-port switch in a small office. The office has 18 PCs, 2 network printers, and a router that provides DHCP and internet access. After the new switch is installed, all link lights are on and users can reach local resources, but none of the PCs receive an IP address automatically. As a result, users cannot access the internet. The router is known to be working correctly. Which of the following is the MOST likely cause?
- A
The uplink from the switch to the router was connected to a VLAN-isolated port
- B
The switch does not support PoE for the printers
- C
The switch is operating at Gigabit speed instead of Fast Ethernet
- D
The PCs require crossover cables to communicate through the new switch
Show answer and explanation
Correct answer: A
Explanation
This question tests practical troubleshooting of switch deployment issues in a small office. When users can communicate locally but cannot obtain DHCP addresses from a router, the technician should suspect a Layer 2 segmentation problem, especially VLAN configuration on a managed switch. DHCP uses broadcast traffic during the discovery process, so clients and the DHCP server or router interface must be in the same broadcast domain unless a DHCP relay is configured. In small office environments, replacing an unmanaged switch with a managed switch can introduce accidental VLAN isolation if ports are not left in the default VLAN or if the uplink is assigned incorrectly. This aligns with standard Ethernet switching behavior and common vendor documentation from Cisco, Netgear, TP-Link, and similar manufacturers regarding VLAN membership and port assignment best practices.
- A. Correct.
Correct. If the port connecting the switch to the router is assigned to the wrong VLAN or isolated from the user ports, devices may still communicate with some local systems on the same switch or VLAN, but DHCP requests will not reach the router. Since DHCP broadcasts must be forwarded within the correct Layer 2 broadcast domain, a VLAN misconfiguration is a realistic and common cause after replacing a switch.
- B. Incorrect.
Incorrect. PoE support affects whether powered devices such as some phones, access points, or cameras receive electrical power over Ethernet. Standard network printers typically use their own power supplies, and lack of PoE would not prevent PCs from receiving DHCP addresses from the router.
- C. Incorrect.
Incorrect. A speed difference such as Gigabit versus Fast Ethernet would normally negotiate automatically on modern Ethernet equipment. Even if a device linked at a lower speed, DHCP would still function as long as Layer 1 and Layer 2 connectivity existed. Speed alone does not explain why every PC fails to obtain an IP address.
- D. Incorrect.
Incorrect. Modern switches and NICs commonly support auto-MDI/MDIX, so straight-through cables are generally used without issue. More importantly, an incorrect cable type would usually prevent link entirely, but the scenario states that link lights are on and local connectivity exists.