Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
interface Vlan30 ip address 10.30.30.1 255.255.255.0 no shutdown interface Vlan99 ip address 10.99.99.1 255.255.255.0 no shutdown DHCP server IP: 10.99.99.20
Clients in VLAN 30 are not receiving addresses from the DHCP server located in VLAN 99. Which configuration change should be made on the Layer 3 interface for VLAN 30?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
Add ip dhcp snooping trust under interface Vlan30.
This is wrong because DHCP snooping trust helps identify trusted ports but does not relay client broadcasts across subnets.
B
Distractor review
Add switchport mode trunk under interface Vlan30.
This is wrong because an SVI is a Layer 3 virtual interface, not a Layer 2 switch port. The missing function is DHCP relay on the client VLAN gateway.
C
Distractor review
Add ip default-gateway 10.99.99.20 under interface Vlan30.
This is wrong because `ip default-gateway` is not used on a Layer 3 SVI to relay DHCP traffic.
D
Best answer
Add ip helper-address 10.99.99.20 under interface Vlan30.
This is correct because the SVI for VLAN 30 is the interface that receives the client DHCP broadcasts. By adding `ip helper-address 10.99.99.20`, the Layer 3 device forwards the request as a unicast packet to the DHCP server in VLAN 99.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is selecting the DHCP snooping trust command or switchport mode trunk on the VLAN interface. DHCP snooping trust is a security feature that designates trusted ports for DHCP messages but does not relay DHCP broadcasts across VLANs or subnets. Similarly, switchport mode trunk applies only to physical Layer 2 switch ports and cannot be configured on Layer 3 SVIs. Another common mistake is using ip default-gateway on the VLAN interface, which is only valid on Layer 2 switches for management traffic and does not enable DHCP relay. These options distract from the correct solution, which is to use ip helper-address to forward DHCP requests to a remote server.
Technical deep dive
How to think about this question
DHCP (Dynamic Host Configuration Protocol) allows clients to obtain IP addresses dynamically. When clients send DHCP requests, they use broadcast messages because they do not yet have an IP address. Broadcasts are limited to the local subnet and do not cross Layer 3 boundaries such as VLAN interfaces or routers. Therefore, when a DHCP server resides on a different VLAN or subnet, the client broadcasts cannot reach it directly. To solve this, Cisco devices use the ip helper-address command on the Layer 3 interface (SVI) that serves as the default gateway for the client VLAN. This command configures the router or Layer 3 switch to forward DHCP broadcast requests as unicast packets to the specified DHCP server IP address. The interface receiving the broadcast converts it and relays it, enabling clients in VLAN 30 to receive addresses from the DHCP server in VLAN 99. Common exam traps include confusing DHCP snooping trust with DHCP relay, or misapplying switchport commands on SVIs. DHCP snooping trust secures DHCP traffic but does not forward broadcasts across VLANs. Similarly, switchport mode trunk is a Layer 2 port configuration and irrelevant for Layer 3 VLAN interfaces. Understanding these distinctions is critical for correctly configuring DHCP relay and ensuring clients receive IP addresses across VLAN boundaries.
KKey Concepts to Remember
- A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
- DHCP client broadcasts do not cross Layer 3 boundaries without a relay agent configured on the VLAN interface.
- The ip helper-address command on a Layer 3 interface forwards UDP broadcasts, including DHCP requests, to a specified DHCP server IP.
- DHCP snooping trust configures trusted ports for DHCP security but does not relay DHCP requests across VLANs or subnets.
- Switchport mode trunk applies only to physical Layer 2 switch ports and is irrelevant for Layer 3 VLAN interfaces (SVIs).
- The ip default-gateway command is used on Layer 2 switches for management traffic and does not enable DHCP relay on Layer 3 interfaces.
- DHCP relay agents convert client broadcast DHCP requests into unicast messages directed to the DHCP server on a different subnet.
- Without ip helper-address, clients in one VLAN cannot obtain IP addresses from a DHCP server located in another VLAN or subnet.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
What is the correct answer to this question?
The correct answer is: Add ip helper-address 10.99.99.20 under interface Vlan30. — The DHCP server is on a different subnet, so the client broadcast messages from VLAN 30 will not naturally cross the Layer 3 boundary. In simple terms, the clients are asking for an address by shouting on their own floor of the building, but the server lives on another floor and cannot hear that broadcast directly. The router or Layer 3 switch must relay the request for them. On Cisco devices, that relay function is usually configured with `ip helper-address` on the interface that receives the client broadcasts. Here, that receiving interface is Vlan30, because that is the default gateway for the clients in VLAN 30. Pointing `ip helper-address` to 10.99.99.20 tells the Layer 3 device to forward DHCP requests to the remote server. DHCP snooping trust is a separate security feature, trunk mode is unrelated to an SVI, and `ip default-gateway` is not the correct solution for relaying DHCP across subnets.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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