DHCP questions cover server configuration, relay agents (ip helper-address), DHCP snooping, and the four-step DORA handshake. Common exam scenarios: a host isn't getting an IP, a relay agent isn't forwarding requests, or a rogue DHCP server is handing out wrong addresses.
Start Scenario PracticeA switch is configured with DHCP snooping and Dynamic ARP Inspection. Hosts suddenly lose connectivity after changing IP settings manually. Which explanation is strongest?
Explanation: The correct answer is A because DAI uses DHCP snooping binding to validate ARP messages; a manual IP change creates a mismatch, causing DAI to block ARP. Option B is incorrect because STP prevents loops and does not block based on MAC address changes. Option C is incorrect because OSPF authentication is a routing protocol feature irrelevant on an access port. Option D is incorrect because PPP authentication applies to serial links, not Ethernet ARP.
A client connects to an employee WLAN using 802.1X authentication. The authentication process completes successfully, but the client fails to obtain an IP address via DHCP. What is the most likely cause?
Explanation: Even after successful 802.1X authentication, the client may be assigned to the wrong VLAN or policy through RADIUS attributes (such as Tunnel-Type or Cisco AV-pair). If that VLAN lacks a DHCP server or correct subnet, the client will not receive an IP address. The other options describe issues that either prevent association entirely (wrong SSID) or are not typical causes in controller-based WLANs (static IP, AP gateway misconfiguration).
A DHCP client on VLAN 30 is not receiving an IP address from a DHCP server (10.99.99.20) on another subnet. The SVI for VLAN 30 is configured with an IP address and is up, but the DHCP relay command is missing. Which command should be added to the SVI configuration?
Explanation: DHCP Discover messages are broadcasts and do not cross routers by default. On an SVI or routed interface facing the clients, an ip helper-address relays those broadcasts to the DHCP server on another subnet.
You are connected to R1 via the console. R1 is the DHCP server for the 192.168.50.0/24 LAN. Configure DHCP on R1 to assign addresses from 192.168.50.10 to 192.168.50.200, with default gateway 192.168.50.1 and DNS server 8.8.8.8. Also, configure R1 to act as a DHCP relay agent for the 10.0.0.0/30 link to reach a remote DHCP server at 203.0.113.10. Then, troubleshoot and fix a misconfiguration that causes clients on VLAN 50 to not receive IP addresses.
Explanation: The DHCP pool is misconfigured: the excluded-address range of 192.168.50.1 through 192.168.50.200 covers most of the pool, but leaves addresses 192.168.50.201 to 192.168.50.254 assignable, which violates the requirement to assign addresses only from 192.168.50.10 to 192.168.50.200. To meet the requirement, you must exclude both the lower range (192.168.50.1 to 192.168.50.9, reserving the gateway) and the upper range (192.168.50.201 to 192.168.50.254). Additionally, the 'ip helper-address' on GigabitEthernet0/0 is unnecessary because R1 itself is the DHCP server for that subnet; it should be removed. The relay agent configuration is missing on the interface facing the remote DHCP server—'ip helper-address 203.0.113.10' should be added to GigabitEthernet0/1 so that broadcasts from the 10.0.0.0/30 subnet are forwarded.
A network administrator has configured a switch port to support a VoIP phone and a desktop PC. Users report that the desktop PC cannot obtain an IP address via DHCP, while the VoIP phone registers successfully. The switch port is up/up, and the desktop is connected to the phone's PC port. What is the most likely cause of the issue?
Explanation: The desktop PC obtains its IP address from the data VLAN, but the switch port's access VLAN is likely misconfigured to the voice VLAN. Changing the access VLAN to the correct data VLAN places the PC in the proper subnet and allows DHCP to function. Option A is unnecessary because a voice-access port does not need to be a trunk. Option C would break the VoIP phone without fixing the PC's VLAN assignment. Option D is irrelevant; PortFast speeds up STP convergence and does not block DHCP.
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Practice all DHCP Troubleshooting ScenariosDHCP questions cover server configuration, relay agents (ip helper-address), DHCP snooping, and the four-step DORA handshake. Common exam scenarios: a host isn't getting an IP, a relay agent isn't forwarding requests, or a rogue DHCP server is handing out wrong addresses. These appear throughout the 200-301 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 200-301. Practicing each scenario type ensures you're ready for any format.
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