The correct configuration involves three fixes: on R1, change the excluded-address range to 192.168.20.1 192.168.20.1 to reserve only the gateway; on SW1, set the ip helper-address under interface Vlan20 to 10.0.0.1; and enable DHCP snooping globally and on VLAN 20, marking the uplink to R1 as trusted. This resolves the core issues because a DHCP server cannot lease addresses from an excluded range, a helper-address must point to the actual server IP, and DHCP snooping blocks rogue servers by treating all ports as untrusted unless explicitly trusted. On the CCNA 200-301 v2 exam, this scenario tests your ability to troubleshoot a multi-layer DHCP deployment, combining server pool logic, relay agent configuration, and security features—a common trap is forgetting that the excluded-address range can block the entire subnet if misconfigured. A useful memory tip: "Exclude only the gate, relay to the right mate, and trust the uplink to keep the rogue out."
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
R1#show running-config | section dhcp
ip dhcp excluded-address 192.168.20.0 192.168.20.255
ip dhcp pool VLAN20_POOL
network 192.168.20.0 255.255.255.0
default-router 192.168.20.1
dns-server 203.0.113.10
!
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
no shutdown
SW1#show running-config | section interface vlan 20
interface Vlan20
ip address 192.168.20.1 255.255.255.0
ip helper-address 192.0.2.99
SW1#show ip dhcp snooping
Global DHCP Snooping is disabled
DHCP Snooping is configured on following vlans:
No VLANs configured
SW1#show interfaces status | include Fa0/3
Fa0/3 connected 1 auto auto 10/100BaseTX
You are connected to R1. Configure R1 as a DHCP server for VLAN 20 clients (192.168.20.0/24) with DNS server 203.0.113.10 and default gateway 192.168.20.1. On switch SW1, enable DHCP snooping globally and on VLAN 20, configure the uplink to R1 as trusted, and ensure that the DHCP server is reachable via ip helper-address on the VLAN 20 SVI. Currently, clients are not receiving IP addresses because of misconfigurations: the excluded-address range on R1 is too large (excluding the entire subnet), the helper-address on SW1 points to a wrong IP (192.0.2.99), and a rogue DHCP server is connected to port Fa0/3 on SW1. Fix all issues so that clients can get addresses securely.
R1#show running-config | section dhcp
ip dhcp excluded-address 192.168.20.0 192.168.20.255
ip dhcp pool VLAN20_POOL
network 192.168.20.0 255.255.255.0
default-router 192.168.20.1
dns-server 203.0.113.10
!
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
no shutdown
SW1#show running-config | section interface vlan 20
interface Vlan20
ip address 192.168.20.1 255.255.255.0
ip helper-address 192.0.2.99
SW1#show ip dhcp snooping
Global DHCP Snooping is disabled
DHCP Snooping is configured on following vlans:
No VLANs configured
SW1#show interfaces status | include Fa0/3
Fa0/3 connected 1 auto auto 10/100BaseTX
A
On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
This option correctly fixes all three issues: the excluded-address range is narrowed to only the gateway, the helper-address points to the correct DHCP server IP (10.0.0.1), and DHCP snooping is enabled with the uplink trusted, blocking the rogue server on Fa0/3.
B
On R1, remove the excluded-address range entirely; on SW1, configure ip helper-address 192.168.20.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
Why wrong: This is incorrect because removing the excluded-address range entirely would allow the DHCP server to assign the gateway IP (192.168.20.1) to a client, causing an IP conflict. Also, the helper-address should point to the DHCP server (10.0.0.1), not the gateway.
C
On R1, change the excluded-address range to 192.168.20.1 192.168.20.254; on SW1, configure ip helper-address 192.0.2.99 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
Why wrong: This is incorrect because the excluded-address range still excludes most of the subnet (only .0 and .255 are available, but .0 is network address and .255 is broadcast), leaving no usable addresses for clients. Also, the helper-address still points to the wrong IP (192.0.2.99).
D
On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, but do not configure any trusted ports.
Why wrong: This is incorrect because without configuring the uplink to R1 as trusted, DHCP snooping will treat all ports as untrusted by default, including the legitimate DHCP server's port. This will cause DHCP offers from the server to be dropped, preventing clients from getting addresses.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
The DHCP server had an excluded-address range covering the entire subnet (192.168.20.0–255), which prevented any IP from being assigned. Fix: change the excluded range to only include the gateway (192.168.20.1) and optionally other reserved addresses. The helper-address on SW1 pointed to 192.0.2.99 (nonexistent) instead of the DHCP server at 10.0.0.1; correct it. DHCP snooping was disabled; enable it globally and on VLAN 20, then configure the uplink to R1 (port connecting to R1) as trusted and the port connecting the rogue server (Fa0/3) as untrusted (or simply leave it untrusted by default, but the rogue server will be blocked).
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
Why this is correct
This option correctly fixes all three issues: the excluded-address range is narrowed to only the gateway, the helper-address points to the correct DHCP server IP (10.0.0.1), and DHCP snooping is enabled with the uplink trusted, blocking the rogue server on Fa0/3.
Related concept
Access ports place end devices into a single VLAN.
✗
On R1, remove the excluded-address range entirely; on SW1, configure ip helper-address 192.168.20.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
Why it's wrong here
This is incorrect because removing the excluded-address range entirely would allow the DHCP server to assign the gateway IP (192.168.20.1) to a client, causing an IP conflict. Also, the helper-address should point to the DHCP server (10.0.0.1), not the gateway.
✗
On R1, change the excluded-address range to 192.168.20.1 192.168.20.254; on SW1, configure ip helper-address 192.0.2.99 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.
Why it's wrong here
This is incorrect because the excluded-address range still excludes most of the subnet (only .0 and .255 are available, but .0 is network address and .255 is broadcast), leaving no usable addresses for clients. Also, the helper-address still points to the wrong IP (192.0.2.99).
✗
On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, but do not configure any trusted ports.
Why it's wrong here
This is incorrect because without configuring the uplink to R1 as trusted, DHCP snooping will treat all ports as untrusted by default, including the legitimate DHCP server's port. This will cause DHCP offers from the server to be dropped, preventing clients from getting addresses.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.Correct answer▾
Why this is correct
This option correctly fixes all three issues: the excluded-address range is narrowed to only the gateway, the helper-address points to the correct DHCP server IP (10.0.0.1), and DHCP snooping is enabled with the uplink trusted, blocking the rogue server on Fa0/3.
✗On R1, remove the excluded-address range entirely; on SW1, configure ip helper-address 192.168.20.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.Wrong answer — click to see why▾
Why this is wrong here
The helper-address must point to the DHCP server, not the default gateway. The excluded-address range should include the gateway to prevent conflicts.
Why candidates choose this
Candidates may think that the gateway IP can be assigned to clients and that the helper-address should be the gateway, confusing the roles of DHCP and routing.
✗On R1, change the excluded-address range to 192.168.20.1 192.168.20.254; on SW1, configure ip helper-address 192.0.2.99 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.Wrong answer — click to see why▾
Why this is wrong here
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.
Why candidates choose this
Candidates might think that excluding a large range is safe or that the helper-address is correct as given, not realizing the IP is incorrect.
✗On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, but do not configure any trusted ports.Wrong answer — click to see why▾
Why this is wrong here
DHCP snooping requires the port connected to the legitimate DHCP server to be explicitly trusted; otherwise, all DHCP server messages are discarded.
Why candidates choose this
Candidates may assume that enabling DHCP snooping globally and per VLAN is sufficient, forgetting to designate trusted ports, or they may think the uplink is automatically trusted.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
Network Services and Security — This question tests Network Services and Security — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted. — The DHCP server had an excluded-address range covering the entire subnet (192.168.20.0–255), which prevented any IP from being assigned. Fix: change the excluded range to only include the gateway (192.168.20.1) and optionally other reserved addresses. The helper-address on SW1 pointed to 192.0.2.99 (nonexistent) instead of the DHCP server at 10.0.0.1; correct it. DHCP snooping was disabled; enable it globally and on VLAN 20, then configure the uplink to R1 (port connecting to R1) as trusted and the port connecting the rogue server (Fa0/3) as untrusted (or simply leave it untrusted by default, but the rogue server will be blocked).
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A client on VLAN 20 must obtain an IPv4 lease from a DHCP server located on VLAN 100. Which feature is required on the Layer 3 interface for VLAN 20?
medium
A.NAT overload
✓ B.DHCP relay
C.Port security
D.Private VLAN
Why B: A DHCP relay agent forwards client broadcasts as unicast to the remote server, typically using ip helper-address.
Variation 2. A router is configured as a DHCP server for VLAN 20. Clients on the VLAN can reach the default gateway, but they do not receive leases. Which two configuration issues on the router would directly prevent successful address assignment?
medium
✓ A.The excluded-address range removes the entire usable subnet
B.The DHCP pool is missing a dns-server statement
C.The subinterface lacks an ip helper-address
✓ D.There are no available addresses left in the defined pool
Why A: The router can serve DHCP locally without an ip helper-address. The real problem is that the excluded-address range consumes every usable host address, leaving the pool with no assignable leases.
Variation 3. A DHCP server is located on a different VLAN from the clients. Which feature is required so the clients can still receive addresses?
medium
A.DHCP snooping
✓ B.DHCP relay
C.Port security
D.NAT overload
Why B: DHCP relay forwards client broadcasts to a remote server as unicast, typically using ip helper-address on the Layer 3 interface.
Last reviewed: Jun 6, 2026
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