The correct configuration requires enabling DHCP snooping globally with `ip dhcp snooping` and per-VLAN with `ip dhcp snooping vlan 20` on MLS1, then setting the uplink port to R1 as trusted via `interface Gig0/0 ip dhcp snooping trust` and applying rate limiting on the access port with `ip dhcp snooping limit rate 10`. This is correct because DHCP snooping acts as a firewall between untrusted access ports and trusted uplink ports, filtering out rogue DHCP server responses and preventing starvation attacks by limiting packet rates. On the CCNA 200-301 v2 exam, this scenario tests your ability to troubleshoot a multi-layered failure: a misconfigured DHCP pool (wrong default-router and DNS), an overly broad excluded-address range that blocks all leases, and a missing security feature. A common trap is forgetting the global `ip dhcp snooping` command—without it, the VLAN-specific command does nothing. Memory tip: “Global first, VLAN second, trust the uplink, rate the access.”
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.
Network Topology
You are connected to the multilayer switch MLS1 in a branch network. The DHCP server on router R1 is supposed to serve the 192.168.20.0/24 VLAN 20, but clients in VLAN 20 are not receiving IP addresses. Additionally, a rogue DHCP server has been detected on VLAN 20. Configure MLS1 to enable DHCP snooping on VLAN 20, set the trust state on the uplink port to R1, and limit the rate of DHCP packets on access ports. Then, on R1, correct the DHCP configuration so that the pool for VLAN 20 uses the correct default-router (192.168.20.1) and DNS server (8.8.8.8), and ensure that the excluded-address range is not too large (exclude only the first 10 addresses). Verify the solution.
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
MLS1# show running-config | section interface
interface GigabitEthernet0/0
description Link to R1
ip address 10.0.0.2 255.255.255.252
no switchport
!
interface GigabitEthernet0/1
description Access port VLAN 20
switchport mode access
switchport access vlan 20
!
interface Vlan20
ip address 192.168.20.1 255.255.255.0
!
MLS1# show ip dhcp snooping
Switch DHCP snooping is disabled
R1# show running-config | section dhcp
ip dhcp excluded-address 192.168.20.1 192.168.20.254
!
ip dhcp pool VLAN20_POOL
network 192.168.20.0 255.255.255.0
default-router 192.168.10.1
dns-server 4.4.4.4
!
R1# show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 10.0.0.1 YES manual up up
A
On MLS1: ip dhcp snooping, ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8
This option correctly enables DHCP snooping on VLAN 20, sets the uplink port as trusted, and applies rate limiting on the access port. It also fixes the DHCP pool with the correct default-router and DNS server, and properly excludes only the first 10 addresses.
B
On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.254, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8
Why wrong: This is incorrect because the excluded-address range excludes all addresses from .1 to .254, leaving only .0 (network) and .255 (broadcast) available, which effectively prevents any dynamic address assignment.
C
On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.10.1 dns-server 4.4.4.4
Why wrong: This is incorrect because the default-router is set to 192.168.10.1 (wrong subnet) and the DNS server is 4.4.4.4 instead of 8.8.8.8. These values do not match the required configuration for VLAN 20.
D
On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8, but no ip dhcp snooping enabled globally on MLS1
Why wrong: This is incorrect because DHCP snooping must be enabled globally with 'ip dhcp snooping' before it can be applied to a VLAN. Without the global command, the VLAN-specific command has no effect.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
On MLS1: ip dhcp snooping, ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8
The problem had three faults: First, the DHCP pool on R1 had a wrong default-router (192.168.10.1 instead of 192.168.20.1) and an incorrect DNS server (4.4.4.4 instead of 8.8.8.8). Second, the excluded-address range was too large (excluding all addresses from .1 to .254 effectively blocked all dynamic assignments; corrected to exclude only .1 through .10). Third, DHCP snooping was disabled on MLS1, allowing a rogue DHCP server. To enable DHCP snooping, both the global `ip dhcp snooping` command and the VLAN-specific `ip dhcp snooping vlan 20` command are required. With snooping enabled, the uplink port Gig0/0 was set as trusted and the access port Gig0/1 was configured with rate limiting to prevent DHCP starvation attacks.
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 MLS1: ip dhcp snooping, ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8
Why this is correct
This option correctly enables DHCP snooping on VLAN 20, sets the uplink port as trusted, and applies rate limiting on the access port. It also fixes the DHCP pool with the correct default-router and DNS server, and properly excludes only the first 10 addresses.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.254, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8
Why it's wrong here
This is incorrect because the excluded-address range excludes all addresses from .1 to .254, leaving only .0 (network) and .255 (broadcast) available, which effectively prevents any dynamic address assignment.
✗
On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.10.1 dns-server 4.4.4.4
Why it's wrong here
This is incorrect because the default-router is set to 192.168.10.1 (wrong subnet) and the DNS server is 4.4.4.4 instead of 8.8.8.8. These values do not match the required configuration for VLAN 20.
✗
On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8, but no ip dhcp snooping enabled globally on MLS1
Why it's wrong here
This is incorrect because DHCP snooping must be enabled globally with 'ip dhcp snooping' before it can be applied to a VLAN. Without the global command, the VLAN-specific command has no effect.
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 MLS1: ip dhcp snooping, ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8Correct answer▾
Why this is correct
This option correctly enables DHCP snooping on VLAN 20, sets the uplink port as trusted, and applies rate limiting on the access port. It also fixes the DHCP pool with the correct default-router and DNS server, and properly excludes only the first 10 addresses.
✗On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.254, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8Wrong answer — click to see why▾
Why this is wrong here
The excluded-address range is too large; it should only exclude the first 10 addresses (1-10) to allow dynamic allocation from .11 onward.
Why candidates choose this
Candidates might think excluding a large range is safe, but it blocks all usable addresses, leaving no addresses for DHCP clients.
✗On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.10.1 dns-server 4.4.4.4Wrong answer — click to see why▾
Why this is wrong here
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.
Why candidates choose this
Candidates may confuse VLAN 20 with VLAN 10 or use a different DNS server, but the question explicitly states the correct values.
✗On MLS1: ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8, but no ip dhcp snooping enabled globally on MLS1Wrong answer — click to see why▾
Why this is wrong here
The global 'ip dhcp snooping' command is required to activate the feature; omitting it leaves DHCP snooping disabled entirely.
Why candidates choose this
Candidates might think enabling snooping on a VLAN is sufficient, but Cisco requires the global enable first.
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.
Trap categories for this question
Command / output trap
This is incorrect because DHCP snooping must be enabled globally with 'ip dhcp snooping' before it can be applied to a VLAN. Without the global command, the VLAN-specific command has no effect.
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 MLS1: ip dhcp snooping, ip dhcp snooping vlan 20, interface Gig0/0 ip dhcp snooping trust, interface Gig0/1 ip dhcp snooping limit rate 10. On R1: ip dhcp excluded-address 192.168.20.1 192.168.20.10, ip dhcp pool VLAN20 network 192.168.20.0 255.255.255.0 default-router 192.168.20.1 dns-server 8.8.8.8 — The problem had three faults: First, the DHCP pool on R1 had a wrong default-router (192.168.10.1 instead of 192.168.20.1) and an incorrect DNS server (4.4.4.4 instead of 8.8.8.8). Second, the excluded-address range was too large (excluding all addresses from .1 to .254 effectively blocked all dynamic assignments; corrected to exclude only .1 through .10). Third, DHCP snooping was disabled on MLS1, allowing a rogue DHCP server. To enable DHCP snooping, both the global `ip dhcp snooping` command and the VLAN-specific `ip dhcp snooping vlan 20` command are required. With snooping enabled, the uplink port Gig0/0 was set as trusted and the access port Gig0/1 was configured with rate limiting to prevent DHCP starvation attacks.
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.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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