Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
Clients: 10.40.40.0/24 in VLAN 40 SVI on distribution switch: 10.40.40.1/24 DHCP server: 172.16.1.10/24 reachable by routing Clients keep sending DHCPDISCOVER and receive no offer.
Exhibit: PCs in VLAN 40 are not receiving addresses from the centralized DHCP server at 172.16.1.10. What should be configured on the VLAN 40 default gateway interface?
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
ip dhcp excluded-address 10.40.40.1 10.40.40.10
Distractor.
B
Best answer
ip helper-address 172.16.1.10
Correct choice.
C
Distractor review
service dhcp
Distractor.
D
Distractor review
ip default-gateway 172.16.1.10
Distractor.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is selecting ip dhcp excluded-address or service dhcp as the solution for DHCP issues across VLANs. The ip dhcp excluded-address command only reserves IP addresses from the DHCP pool and does not forward DHCP requests to a server on another subnet. The service dhcp command enables the router to act as a DHCP server itself, which is unnecessary if a centralized DHCP server exists. Another common mistake is confusing ip default-gateway with DHCP relay; ip default-gateway sets a device’s default route and does not forward DHCP broadcasts. Candidates must recognize that only ip helper-address enables the router to relay DHCP broadcasts between different subnets.
Technical deep dive
How to think about this question
DHCP (Dynamic Host Configuration Protocol) is a network management protocol used to dynamically assign IP addresses and other network configuration parameters to devices on a network. DHCP clients initially send broadcast messages to discover DHCP servers. However, broadcasts are limited to the local subnet and do not traverse routers by default. When clients and the DHCP server reside on different subnets, a mechanism is required to forward these broadcasts across subnet boundaries. The ip helper-address command on a router interface configured as a VLAN default gateway enables DHCP relay. This command instructs the router to forward DHCP broadcast packets received on that interface to the specified DHCP server IP address, in this case, 172.16.1.10. The router converts the broadcast into a unicast message directed at the DHCP server, allowing clients in VLAN 40 to obtain IP addresses from the centralized DHCP server despite being on a different subnet. A common exam trap is confusing DHCP relay with DHCP server configuration or IP address exclusion. Commands like ip dhcp excluded-address only reserve IP addresses from the DHCP pool and do not facilitate relay. Similarly, service dhcp enables the router to act as a DHCP server but does not forward requests to another server. The ip default-gateway command sets a device's default route and does not influence DHCP traffic. Understanding the role of ip helper-address in relaying DHCP requests is critical for correctly configuring VLAN interfaces in multi-subnet environments.
KKey Concepts to Remember
- A router interface configured as a VLAN default gateway uses the ip helper-address command to forward DHCP broadcasts to a DHCP server on a different subnet.
- DHCP clients send broadcast messages that cannot cross router boundaries without relay configuration, so ip helper-address enables DHCP relay functionality.
- The ip helper-address command specifies the IP address of the centralized DHCP server to which the router forwards DHCP requests.
- Without ip helper-address, DHCP broadcasts from clients in VLAN 40 remain local and do not reach the DHCP server at 172.16.1.10 on another subnet.
- The ip dhcp excluded-address command prevents the DHCP server from assigning specific IP addresses within a subnet but does not enable DHCP relay.
- The service dhcp command enables the router to act as a DHCP server, which is unrelated to relaying DHCP requests to a centralized server.
- The ip default-gateway command configures the default gateway for a device and does not affect DHCP relay or address assignment.
- Routers must be explicitly configured with ip helper-address on VLAN interfaces to relay DHCP broadcasts across different IP subnets.
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 router interface configured as a VLAN default gateway uses the ip helper-address command to forward DHCP broadcasts to a DHCP server on a different subnet.
What is the correct answer to this question?
The correct answer is: ip helper-address 172.16.1.10 — When DHCP clients and the DHCP server are on different subnets, the router interface serving the client subnet must relay broadcasts to the server with the ip helper-address command.
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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