CCNA DHCP Questions

Incorrect because the dns-server statement is optional; its absence does not prevent the DHCP server from assigning IP addresses to clients.

Incorrect because the ip helper-address command is only necessary when forwarding DHCP requests across subnets, not when the router itself is the DHCP server on the VLAN.

Using a /16 subnet mask (255.255.0.0) does not match the corporate /24 network, leading to incorrect network identification and potential routing issues.

Leaving the default gateway blank prevents the host from reaching any network beyond its own subnet, so internet access is impossible.

Setting the default gateway to the host's own IP address (192.168.1.50) would cause traffic to be sent to itself, never reaching the actual gateway.

PortFast is a Spanning Tree Protocol feature that immediately transitions a port to forwarding state, bypassing listening and learning states. It does not perform any validation of ARP packets or prevent IP-to-MAC spoofing attacks.

EtherChannel is a link aggregation technology that combines multiple physical links into a single logical link to increase bandwidth and provide redundancy. It does not inspect or validate ARP packets, so it cannot prevent IP-to-MAC spoofing.

NetFlow is a network monitoring protocol that collects IP traffic statistics for analysis and troubleshooting. It provides visibility into traffic patterns but does not actively block or validate ARP packets, so it cannot prevent spoofing attacks.

Candidates often jump to switch security features when they see DHCP issues, but without evidence of a switch misconfiguration, it is not the most direct next action.

Test-takers often consider firewalls as universal blockers, forgetting that in a flat Layer 2 domain, broadcast traffic such as DHCP Discover is not filtered by a firewall.

Candidates prefer simple client-side fixes, but CCNA troubleshooting emphasises identifying root causes before applying band-aid solutions.

ping 127.0.0.1 only tests the local TCP/IP stack and does not verify DHCP address assignment or DNS resolution.

tracert 8.8.8.8 uses an IP address directly and does not test domain-name resolution.

arp -a displays the ARP cache, which is unrelated to DHCP or DNS.

The network statement is already correct; changing it does not resolve the incorrect default gateway.

No helper address is needed because the DHCP server (the router itself) is on the same subnet as the PC.

Setting the interface IP to 192.168.1.254 would create an address mismatch with the DHCP pool’s gateway and break the subnet.

The network command in DHCP pool configuration requires a subnet mask in dotted decimal format, not CIDR notation like /24.

The lease command uses days as its unit; `lease 24` sets a 24-day lease, not the required 1 day.

The lease command sets duration in days, so `lease 24` gives a 24-day lease instead of a 1-day lease.

NAT overload translates private IP addresses to a public IP for outbound traffic but does not forward DHCP broadcasts between VLANs. It is unrelated to DHCP relay functionality required for inter-VLAN DHCP communication.

Port security restricts which MAC addresses can connect to a switch port but does not forward DHCP broadcasts or relay DHCP messages between VLANs, so it cannot enable DHCP communication across VLANs.

Private VLANs isolate devices within a VLAN for security purposes but do not provide DHCP relay capabilities or forward DHCP requests between VLANs, making this option incorrect.

The 'ip dhcp excluded-address' command is used on a DHCP server to prevent certain addresses from being assigned, not on a router interface to forward DHCP requests. This command would not help clients in VLAN 40 reach the centralized DHCP server.

The 'service dhcp' command globally enables the DHCP server or relay agent on a Cisco device, but it does not specify where to forward requests. Without the 'ip helper-address' command, DHCP broadcasts will not be forwarded to the server.

The 'ip default-gateway' command is used on a switch to set a default gateway for management purposes, not to forward DHCP broadcasts. It does not provide DHCP relay functionality.

Disabling DAI on all VLANs globally removes ARP spoofing protection network-wide, which is excessive and reduces security unnecessarily. The question requires a targeted fix, so this option is incorrect.

Trusting the user-facing printer access port for DHCP snooping and DAI is too permissive and can allow malicious ARP packets, weakening security. It is not recommended as a best practice or exam answer.

Changing the printer to use a larger MTU does not affect ARP packet validation or DAI behavior. MTU size is unrelated to ARP inspection, so this option is irrelevant and incorrect.

SLAAC does not require NAT because IPv6 has a vast address space, eliminating the need for address translation. NAT is a workaround for IPv4 address exhaustion and is not used in native IPv6 networks. SLAAC relies on router advertisements to provide prefix information, and hosts generate their own addresses without any translation.

DHCPv6 does not replace router advertisements; in fact, router advertisements are still required for hosts to determine the default gateway and other network parameters. Even when DHCPv6 is used, hosts rely on RAs to learn the on-link prefix and to decide whether to use stateful or stateless configuration.

Both SLAAC and DHCPv6 can provide addressing information to hosts. SLAAC allows hosts to form their own addresses from prefix information in RAs, while DHCPv6 can assign addresses and other configuration parameters. Therefore, the statement that neither can provide addressing information is false.

The relay address 192.168.100.10 is correctly configured to point to the DHCP server on VLAN 100. The issue is not with the relay address being incorrect.

The DHCP server is on the same router (VLAN 100 interface) and is reachable; the relay configuration is correct. The server is not unreachable.

The exhibit shows 'ip helper-address 192.168.100.10' is configured on GigabitEthernet0/1, which is the VLAN 200 interface. The command is present.

The ip helper-address must be configured on the interface that receives the client broadcasts (VLAN 20), not the server's VLAN.

This command enables relay agent information insertion, not the actual forwarding of DHCP packets to a server.

The question states the DHCP server is at 192.168.10.100, so R1 should relay, not serve.

The `ip helper-address` command is configured on the client-facing interface, not the server-facing interface.

The command does not forward all UDP broadcasts; it only forwards specific UDP ports (DHCP, DNS, TFTP, etc.) by default.

The command does not validate source MAC addresses or prevent DHCP spoofing; it only relays DHCP messages.

This statement incorrectly associates two separate features; DHCP snooping operates at Layer 2 for DHCP security, not for CDP.

This function belongs to the relay agent, not to DHCP snooping, which operates within a single VLAN to enforce security policies.

This is a relay agent feature; DHCP snooping does not alter the broadcast nature of DHCP packets, it only filters them.

Candidates may think that an empty binding table means the service is off, but the command itself would fail or show a service status if DHCP were truly disabled.

Candidates often associate 'no addresses assigned' with a pool misconfiguration, but a pool mismatch would not prevent the server from seeing DISCOVER messages and possibly creating failed or erroneous bindings.

Candidates may confuse a missing option with a complete failure of DHCP operation, but IP address leasing is independent of options like default-router.

PortFast is a Spanning Tree Protocol feature that immediately transitions an access port to the forwarding state, bypassing the listening and learning states. It does not inspect or validate DHCP messages, so it cannot prevent a rogue DHCP server from handing out addresses.

HSRP (Hot Standby Router Protocol) provides first-hop redundancy by allowing multiple routers to share a virtual IP address, ensuring gateway availability. It does not inspect DHCP traffic or provide any mechanism to block unauthorized DHCP servers.

LLDP (Link Layer Discovery Protocol) is a vendor-neutral protocol used for discovering neighboring devices and their capabilities by exchanging information such as device type, management addresses, and VLAN IDs. It has no role in DHCP security or filtering DHCP messages.

Changing VLANs or using subinterfaces does not solve the problem because the router's physical interface must be up and have an IP address to serve the VLAN.

Enabling DHCP snooping or trust settings on the switch cannot fix a router interface that is administratively down and unassigned.

Assigning a static IP to the PC circumvents but does not resolve the root issue of the router's interface being down, and the scenario requires DHCP.

STP (Spanning Tree Protocol) prevents loops in redundant topologies and does not block hosts based on IP or MAC address changes. It operates at Layer 2 and is unrelated to IP address configuration.

OSPF is a Layer 3 routing protocol used between routers, not on access ports connecting hosts. OSPF authentication is configured on router interfaces and does not apply to host ARP traffic on a switch.

PPP (Point-to-Point Protocol) authentication is used on serial links or PPPoE connections, not on Ethernet switch ports. It is unrelated to ARP inspection or DHCP snooping in a switched network.

This distractor exploits the common tendency to blame the helper address configuration first, overlooking the security feature that silently drops the returning DHCPOFFER.

This plays on the misconception that a switch requires an IP address on the client VLAN to facilitate DHCP, when in fact the router acting as the relay agent provides Layer 3 connectivity.

This misinterprets DHCP snooping behavior: it assumes all DHCP traffic is filtered on untrusted ports, overlooking the critical distinction that only server-side messages are blocked, not client requests.

The PC's APIPA address cannot reach the 192.168.10.1 gateway because it is not in the same subnet, so configuring a default gateway alone does not restore connectivity.

Changing the VLAN to VLAN 1 would isolate PC-A from the correct subnet (VLAN 10), breaking connectivity instead of fixing it.

The 'ip helper-address' command is used on interfaces that need to forward DHCP broadcasts to a remote DHCP server; here R1 itself is the DHCP server and is directly attached, so the command is not needed.

An incorrect SSID would prevent the client from associating to the WLAN at all, not allow authentication followed by DHCP failure.

A statically configured IP is less likely the strongest cause because the client would either not use DHCP or would obtain an APIPA address, and the issue is specifically about failing to obtain a correct address via DHCP.

The access point’s default gateway does not impact DHCP forwarding for client traffic; in controller-based deployments, DHCP packets are handled by the controller or bridged directly to the wired network.

This skips the obvious configuration mismatch and targets the wrong component. It assumes the issue is on the server side rather than the router's DHCP relay placement.

This action investigates Layer 1/2 status when the problem is already identified as a Layer 3 (DHCP relay) configuration error. It skips applying the fix and delays resolution.

Candidates might think the command simply didn't take effect and that reapplying it solves the problem, misunderstanding the directional requirement of DHCP relay placement.

Many engineers mistakenly think the helper should reside on the server VLAN; however, it must reside on the client-facing L3 interface.

A common misconception is that DHCP relay bypasses snooping, but snooping still inspects the server's unicast response and drops it unless the ingress port is trusted.

Candidates often suspect routing issues, but verified reachability eliminates this. The problem lies in the snooping policy, not IP connectivity.

Candidates often prematurely blame security features when DHCP fails, ignoring the pool statistics right in front of them.

A common mistake is assuming that conflicts always fill up a pool, but the zero value directly disproves this.

Misunderstanding subnet sizing often leads candidates to blame the mask, but the exhibit confirms the mask is appropriate for the pool size.

Option A incorrectly states that the ip helper-address converts DHCP unicast replies into broadcasts. The command actually relays client broadcast requests as unicast to the server, not the reverse. This reverses the direction of the relay function and misunderstands the broadcast boundary issue.

Option C incorrectly claims the command provides DNS resolution for DHCP clients before they receive an address. DNS resolution is unrelated to the ip helper-address function, which only relays UDP broadcasts like DHCP requests and does not perform name resolution.

Option D mistakenly suggests that the command creates a static route to the DHCP server. Routing and static routes are separate functions; the ip helper-address does not affect routing tables but only relays broadcast traffic as unicast.

The DISCOVER message is used by the client to locate available DHCP servers, not to accept an offered address. It is the first step in the DORA process.

The OFFER message is sent by the DHCP server to propose an IP address to the client, not by the client to accept it. The client cannot send an OFFER.

The ACK message is sent by the DHCP server to confirm the lease after receiving the REQUEST, not by the client. The client does not send ACK.

The helper-address must be a unicast IP address of the DHCP server, not a broadcast address. Additionally, only ports connected to legitimate DHCP servers should be trusted; all other ports must be untrusted to block rogue servers.

The excluded-address range should only reserve a few addresses (e.g., for the gateway and static assignments), not the entire subnet. The port connected to the DHCP server must be trusted to allow DHCP server messages; untrusted ports block such messages.

DHCP snooping requires that ports connected to legitimate DHCP servers be configured as trusted. Untrusted ports are for client-facing ports where rogue servers might appear; they drop DHCP server messages.

PortFast is a feature that allows switch ports to bypass the usual STP listening and learning states to quickly transition to forwarding. It does not perform any ARP packet validation or security checks, so it cannot prevent ARP spoofing.

UDLD (Unidirectional Link Detection) is designed to detect and disable unidirectional links between switches to prevent network loops or blackholes. It does not inspect or validate ARP packets and thus does not stop ARP spoofing.

HSRP preemption is a feature related to first-hop redundancy protocols that allows a higher priority router to take over as the active gateway. It does not provide any ARP packet validation or protection against ARP spoofing.

The excluded-address range is too large; it should only exclude the first 10 addresses (1-10) to allow dynamic allocation from .11 onward.

The default-router must be the gateway for VLAN 20 (192.168.20.1), and the DNS server should be 8.8.8.8 as specified.

The global 'ip dhcp snooping' command is required to activate the feature; omitting it leaves DHCP snooping disabled entirely.

Incorrect DNS settings prevent name resolution but do not affect IP address assignment. The workstation would still receive a valid IP from DHCP, not an APIPA address.

If the DHCP pool were exhausted, all workstations would fail to obtain addresses and use APIPA. The scenario states other workstations are working correctly, so pool exhaustion is not the cause.

A misconfigured default gateway would prevent internet access but the workstation would still receive a valid IP from DHCP. APIPA addresses are only assigned when DHCP fails entirely.

DHCP does not translate private addresses to public; that is the function of NAT.

DHCP does not replace the need for subnet masks and default gateways; it actually provides them automatically.

DNS and DHCP operate at the Application Layer (Layer 7), not Layer 1.

Incorrect. The command switchport mode trunk is a Layer 2 switchport configuration and cannot be applied to an SVI, which is a Layer 3 interface. This does not affect DHCP relay.

Incorrect. The ip default-gateway command sets the default gateway for management traffic on a Layer 2 device and does not relay DHCP broadcasts. It does not restore DHCP service for clients.

Incorrect. The spanning-tree portfast command is used on physical Layer 2 switchports to speed up port transitions and has no effect on DHCP relay or SVIs.

PortFast is used to speed up the transition of a port to forwarding state in spanning tree, typically for end-user devices. It does not filter DHCP messages or prevent rogue DHCP servers; DHCP snooping is the correct mechanism.

ARP is essential for IP communication and disabling it would break network connectivity. DHCP snooping does not involve ARP; it operates at the DHCP protocol level to validate messages.

Setting every access port as trusted would allow any device connected to those ports to act as a DHCP server, defeating the purpose of DHCP snooping. Only ports connected to legitimate DHCP servers should be trusted.

Many candidates confuse conflict detection with a preventive mechanism, thinking that enabling it would stop the assignment of an already-used address.

Candidates may associate short leases with instability, but the root cause is the missing exclusion, not the lease timer.

Beginners often conflate the default gateway with the assigned IP address, believing a mistake in the default-router setting could cause an address conflict.

An STP root bridge ID is irrelevant to host IP reachability because it pertains to Layer 2 spanning tree topology and does not affect IP routing or DHCP configuration for hosts.

A voice VLAN value is specific to voice traffic segmentation on switches and does not influence a host’s ability to route IP packets to remote networks, making it unrelated to the connectivity issue.

A router ID is a concept used in routing protocols like OSPF and does not apply to DHCP or host IP configuration, so it cannot cause the described connectivity problem.

Candidates may confuse administratively excluded addresses with dynamically detected conflicts.

The misconception is that aggressive ping settings create false conflicts, when in fact a conflict entry proves a reply was received.

Candidates might think that conflicts represent stale entries, but a conflict is a permanent record of a detected collision, not a lease state.

The ip default-gateway command configures the default gateway for a Layer 2 switch’s management interface and does not forward DHCP broadcasts. Since the question involves DHCP relay across routed VLANs, this command is irrelevant.

The switchport trunk allowed vlan 20 command controls VLAN traffic allowed on a trunk link but does not influence DHCP relay or routing between VLANs. The issue is DHCP relay, not VLAN trunk configuration.

The spanning-tree portfast default command enables PortFast on switch ports to speed up STP convergence and does not affect DHCP relay or routing. It is unrelated to the problem of clients not receiving DHCP addresses.

Misunderstanding ACL processing—assumes a simple subnet permit ACL would block port 22 by default, but the ACL entry permits all traffic from the subnet, not just specific ports.

Assumes port security is the first cause of connectivity failure when MAC limits are configured, but the symptom does not indicate a port security violation; the port would need to go into err-disabled, which is not mentioned.

AAA failures manifest as authentication errors or prompts that time out after attempting RADIUS, but they typically affect all attempts, not a single source, unless combined with ACLs that permit other hosts but block this one.

Candidates may think that a large number of untrusted hosts could overwhelm the binding table, but static hosts do not interact with DHCP and would not fill the table or be rejected.

Candidates often confuse DAI and IP Source Guard since both use DHCP snooping; however, DAI specifically validates ARP packets, which matches the symptom of connectivity loss due to ARP resolution failure.

Some candidates might associate ARP security features with port shutdown, but standard DAI operation does not disable ports, and the symptom does not indicate interface down events.

This is a distractor term that does not exist in the DHCP snooping configuration. The correct states are trusted and untrusted.

This term is borrowed from other security contexts and is not applicable to DHCP snooping trust states.

This confuses the DHCP relay agent feature with DHCP snooping trust states. They are different mechanisms.

Incorrect. Layer 3 route loops are routing protocol issues and are not addressed by DHCP snooping, which operates at Layer 2 for DHCP message validation.

Incorrect. Trunk encapsulation mismatches relate to VLAN tagging and trunk negotiation, which DHCP snooping does not influence or prevent.

Incorrect. Weak SSH ciphers pertain to encryption security and have no connection to DHCP snooping, which focuses solely on DHCP message filtering.

The ip directed-broadcast command enables forwarding of directed broadcasts but does not relay DHCP requests. It is unrelated to DHCP relay and will not solve the problem of clients not receiving addresses from a remote DHCP server.

service dhcp-server is not a valid Cisco IOS interface command for DHCP relay. It does not configure the router to forward DHCP broadcasts and thus will not resolve the issue.

default-router is a DHCP pool parameter used to assign the default gateway IP address to clients. It does not configure the interface to relay DHCP broadcasts and is not relevant to the relay configuration.

The SVI must have an IP address for routing, but the DHCP server can serve addresses from any pool regardless of the SVI's subnet. The issue is with the pool itself.

If DHCP snooping were blocking messages, the 'Malformed messages' counter might increase, but the server would still receive some messages. The zero count indicates no messages are reaching the server.

There is no 'vlan' command under a DHCP pool. The pool serves all hosts in the subnet regardless of VLAN association.

This option is incorrect because DHCP does not replace DNS. DNS is a separate service responsible for name resolution, while DHCP only provides IP configuration parameters including DNS server addresses.

This option is wrong because DHCP does not remove the need for default gateways. Hosts still require gateway information to communicate outside their subnet, and DHCP typically provides this information during configuration.

This option is incorrect because routing protocols do not depend on DHCP. Routing protocols function independently of IP address assignment methods and do not require DHCP to operate on user networks.

This option is incorrect because ARP is used for local subnet communication and does not depend on the presence of a default gateway. The host can still use ARP to communicate locally.

This option is wrong because a host does not automatically become part of every remote subnet without a gateway. The subnet mask defines the local subnet boundaries, and gateway absence does not change this.

This option is unrelated to the problem. Converting an access port to a trunk port affects VLAN tagging on switches but does not impact a host’s ability to have or use a default gateway.

Duplicate IP conflicts typically result in a warning but do not cause the system to assign a 169.254.x.x address.

DNS issues do not affect IP address assignment via DHCP.

The subnet mask is correct for the APIPA range; the problem is the lack of a DHCP server response.

The helper-address must be the IP of the DHCP server, which is the SVI address 192.168.50.1, not the link address 10.0.0.1.

The helper-address misconfiguration prevents DHCP requests from being forwarded to the correct server, so fixing only the excluded range is insufficient.

The excluded range must be corrected to allow hosts to receive addresses from the pool; otherwise, the DHCP server will not assign addresses.

The DNS server option is not critical for basic connectivity to remote networks; it primarily affects name resolution. If a client can obtain an IP address but cannot reach remote networks, the issue is more likely related to the default gateway configuration.

The lease time option specifies how long a DHCP lease is valid, but it does not affect the ability to reach remote networks. Therefore, a missing or incorrect lease time would not directly cause connectivity issues.

The TFTP server option is not necessary for a client to reach remote networks, as it primarily facilitates file transfers and does not impact routing or network accessibility. Therefore, its absence would not directly cause connectivity issues to remote networks.

This option is wrong because enabling DHCP snooping trust on VLAN 30 does not facilitate communication with the DHCP server in VLAN 99; it only protects against rogue DHCP servers.

Adding 'switchport mode trunk' under interface Vlan30 is incorrect because VLAN 30 is already configured as a Layer 3 interface, and trunking is not applicable to Layer 3 interfaces. This command is used for Layer 2 interfaces to allow multiple VLANs over a single link.

This option is wrong because the command 'ip default-gateway' is used to set a default gateway for a Layer 2 device, not for enabling DHCP relay on a Layer 3 interface. Clients in VLAN 30 need a helper address to reach the DHCP server in VLAN 99.

The helper-address must point to the DHCP server, not the default gateway. The excluded-address range should include the gateway to prevent conflicts.

The excluded-address range must be limited to reserved addresses only, not the entire usable range. The helper-address must be corrected to 10.0.0.1.

DHCP snooping requires the port connected to the legitimate DHCP server to be explicitly trusted; otherwise, all DHCP server messages are discarded.

It is a supporting feature for relay agents, not the primary command to forward DHCP broadcasts.

It is a prerequisite but not sufficient; the ip helper-address is still needed to direct the traffic.

The command does not exist; it is a fabrication.

The specific factual error: Excluding 10 addresses does not exhaust the pool; the pool still has 244 addresses available.

The specific factual error: The pool configuration is already correct; modifying it does not address the disabled DHCP service.

The specific factual error: 'ip helper-address' is for DHCP relay across subnets; it is not needed when the server is local.

This option assumes that the loss of DHCP service is due to a missing relay agent, but the symptom started only after enabling snooping, not after an infrastructure change that would affect the relay path.

The idea that a missing database causes immediate traffic blocking is a common misinterpretation of the database’s role—it is purely for persistency, not for runtime filtering.

Many candidates assume that DHCP snooping uses a reverse-check against the binding table for any DHCP server messages, but the filtering is based solely on the trusted/untrusted port state, not on a learned server entry.

The specific factual error is that the access port (G0/1) should be untrusted, not trusted. Only the uplink port to the legitimate DHCP server should be trusted.

The specific factual error is that the ip helper-address must be set to the actual DHCP server IP (10.0.0.2), not 10.0.0.3.

The specific factual error is that only the uplink port (G0/0) should be trusted; trusting the access port (G0/1) allows any device connected to that port to act as a DHCP server.

A static IP on the server does not forward broadcasts across subnets; the server's reachability is not the issue.

Enabling DHCP snooping without proper configuration could block legitimate DHCP traffic, and it does not solve the broadcast forwarding issue.

The problem is not about lease duration; clients cannot receive any DHCP offers because broadcasts are not forwarded.