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 PracticeWhich security feature is used to mitigate DHCP starvation attacks by limiting the number of DHCP messages per port?
Explanation: DHCP starvation attacks deplete the available IP address pool by requesting numerous leases. DHCP Snooping protects the network by acting as a firewall between untrusted hosts and DHCP servers, limiting the rate of DHCP messages. This is a critical enterprise security control, as DHCP is often an overlooked vector that can lead to DoS or man-in-the-middle attacks if left entirely unprotected on access-layer switches.
A network engineer is troubleshooting a potential DHCP issue. Which THREE components are provided to a client during a successful DHCP 'Acknowledgment' (ACK) message? (Choose three.)
Explanation: The DHCP ACK is the final step in the DORA process where the server confirms the lease of an IP address. Understanding the specific parameters delivered in this message is crucial for diagnosing configuration errors that affect host connectivity, name resolution, and the ability of devices to reach resources outside their local network segment.
Refer to the exhibit. What is the effect of the 'ip dhcp snooping trust' command on the designated interface?
Explanation: In DHCP Snooping, ports are untrusted by default. The 'trust' command marks an interface as connected to a legitimate DHCP server, allowing DHCPOFFER and DHCPACK messages to pass through. Without this command, the switch would drop these server-originated messages on that port, preventing clients from receiving IP addresses. This is critical for controlling which DHCP servers are allowed to interact with the network, thereby mitigating unauthorized server attacks.
An administrator wants to ensure that only legitimate DHCP servers provide IP addresses to clients on the network. Which feature should be enabled?
Explanation: Rogue DHCP servers can be used to launch man-in-the-middle attacks by providing clients with malicious gateway addresses. DHCP Snooping creates a distinction between trusted and untrusted ports. By designating only ports connected to authorized servers as trusted, the network automatically blocks DHCP response traffic from unauthorized sources. This is essential for maintaining control over the network's address allocation process and preventing redirection of user traffic.
A technician is troubleshooting a workstation that cannot connect to the network. The workstation has a link light, but ipconfig shows an IP address of 169.254.10.20. The technician suspects a DHCP-related issue. Which two commands should the technician use to attempt to obtain a new IP address from the DHCP server? (Choose two.)
Explanation: An IP address in the 169.254.x.x range indicates that the workstation failed to obtain a lease from a DHCP server and self-assigned an APIPA address. To resolve this, the technician should first release the current APIPA address using ipconfig /release, then request a new lease with ipconfig /renew. These two commands together attempt to re-establish DHCP communication and obtain a valid IP configuration.
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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 100-150 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 100-150. Practicing each scenario type ensures you're ready for any format.
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