The correct answer is that the traffic is permitted because the host’s IP falls within the allowed subnet and its MAC is validated by port security. IP Source Guard typically filters traffic based on DHCP snooping bindings, but when a static IP host is involved, IPSG can be configured with a static binding or, as in this scenario, rely on the fact that the source IP (10.1.1.1) matches the configured subnet (e.g., 10.0.0.0/8) and the MAC address is a secure MAC learned by port security. On the ENCOR 350-401 exam, this tests your understanding of how Layer 2 security features interact—specifically that IPSG does not blindly block all non-DHCP traffic; it can coexist with port security to permit static hosts. A common trap is assuming IPSG always requires a DHCP binding, but the key insight is that a valid subnet and a secure MAC can satisfy both features. Memory tip: “Subnet plus secure MAC equals IPSG green light.”
350-401 Security Practice Question
This 350-401 practice question tests your understanding of 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
Refer to the exhibit.
interface GigabitEthernet0/1
ip access-group ACL-IN in
ip verify source port-security
!
ip access-list extended ACL-IN
permit tcp 10.0.0.0 0.255.255.255 any eq 80
permit tcp 10.0.0.0 0.255.255.255 any eq 443
deny ip any any
Refer to the exhibit. A switch has IP Source Guard (IPSG) and port-security enabled on interface GigabitEthernet0/1. A host with IP 10.1.1.1 and MAC 00:1A:2B:3C:4D:5E is connected and tries to access a web server at 192.168.1.100. What will happen?
Refer to the exhibit.
interface GigabitEthernet0/1
ip access-group ACL-IN in
ip verify source port-security
!
ip access-list extended ACL-IN
permit tcp 10.0.0.0 0.255.255.255 any eq 80
permit tcp 10.0.0.0 0.255.255.255 any eq 443
deny ip any any
A
The traffic is blocked because the host is not using DHCP, so IPSG drops all non-DHCP traffic.
Why wrong: IPSG can work with static bindings; not necessarily blocked.
B
The traffic is permitted only if the destination is also in the 10.0.0.0/8 range.
Why wrong: ACL permits to any destination.
C
The traffic is blocked because IP Source Guard requires a static binding for the host.
Why wrong: IPSG uses DHCP snooping bindings; static not required.
D
The traffic is permitted because the host's IP is within the allowed subnet and the MAC is valid according to port-security.
Correct: IP source guard checks that the source IP is in the binding table; if valid, traffic passes ACL.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The traffic is permitted because the host's IP is within the allowed subnet and the MAC is valid according to port-security.
Option D is correct because IP Source Guard (IPSG) on a switch port typically uses DHCP snooping bindings to validate traffic. However, when port-security is also enabled and the host's IP (10.1.1.1) falls within the configured subnet (e.g., 10.0.0.0/8), and the MAC address (00:1A:2B:3C:4D:5E) matches a port-security secure MAC address, the switch can permit the traffic. IPSG does not inherently block all non-DHCP traffic; it can be configured with static bindings or rely on DHCP snooping, but in this scenario, the combination of a valid subnet and port-security allows the traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The traffic is blocked because the host is not using DHCP, so IPSG drops all non-DHCP traffic.
Why it's wrong here
IPSG can work with static bindings; not necessarily blocked.
✗
The traffic is permitted only if the destination is also in the 10.0.0.0/8 range.
The traffic is blocked because IP Source Guard requires a static binding for the host.
Why it's wrong here
IPSG uses DHCP snooping bindings; static not required.
✓
The traffic is permitted because the host's IP is within the allowed subnet and the MAC is valid according to port-security.
Why this is correct
Correct: IP source guard checks that the source IP is in the binding table; if valid, traffic passes ACL.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that IPSG always requires DHCP snooping and blocks all non-DHCP traffic, but in reality, IPSG can be configured with port-security to allow traffic from statically assigned hosts within a valid subnet.
Detailed technical explanation
How to think about this question
IP Source Guard works by maintaining a per-port IP-to-MAC binding table derived from DHCP snooping or static ip source binding commands. When a host sends traffic, the switch checks the source IP and MAC against this table; if no match exists, the packet is dropped. Port-security adds an additional layer by validating the source MAC against learned or configured secure MAC addresses. In this scenario, the host's IP is within the allowed subnet (10.0.0.0/8), and the MAC is valid per port-security, so the switch permits the traffic even without a DHCP snooping entry, as long as the IP subnet check passes (if configured with ip verify source port-security).
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The traffic is permitted because the host's IP is within the allowed subnet and the MAC is valid according to port-security. — Option D is correct because IP Source Guard (IPSG) on a switch port typically uses DHCP snooping bindings to validate traffic. However, when port-security is also enabled and the host's IP (10.1.1.1) falls within the configured subnet (e.g., 10.0.0.0/8), and the MAC address (00:1A:2B:3C:4D:5E) matches a port-security secure MAC address, the switch can permit the traffic. IPSG does not inherently block all non-DHCP traffic; it can be configured with static bindings or rely on DHCP snooping, but in this scenario, the combination of a valid subnet and port-security allows the traffic.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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