The answer is that the host can communicate only with the promiscuous port. This is because in a private VLAN configuration, a host placed in an isolated secondary VLAN—such as secondary PVLAN 200 associated with primary PVLAN 100—cannot directly communicate with any other host within the same primary VLAN, regardless of whether those hosts are in community or other isolated VLANs. The only permitted traffic path is to the promiscuous port, which typically connects to a router or firewall and handles inter-VLAN routing. On the CISSP exam, this concept tests your understanding of network segmentation and isolation controls, often appearing in questions about preventing lateral movement within a VLAN. A common trap is assuming that all hosts in the same primary VLAN can communicate, but private VLANs override that default behavior. Memory tip: think of the isolated port as a “dead end” that can only talk to the promiscuous “gatekeeper.”
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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. The following is a configuration snippet from a network device:
interface GigabitEthernet0/1
switchport mode private-vlan host
switchport private-vlan host-association 100 200
A network administrator has configured private VLANs on a switch. The host in this port is part of PVLAN 100, and its associated secondary PVLAN is 200. What is the expected behavior for traffic from this host to other hosts in the same primary VLAN 100?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Refer to the exhibit. The following is a configuration snippet from a network device:
interface GigabitEthernet0/1
switchport mode private-vlan host
switchport private-vlan host-association 100 200
A
It cannot communicate with any other host
Why wrong: It can communicate with the promiscuous port.
B
It can communicate only with other hosts in the same secondary VLAN 200
Why wrong: Host ports in private VLANs cannot communicate with other host ports, regardless of secondary VLAN.
C
It can communicate with the promiscuous port only
Private VLAN host ports can only communicate with the promiscuous port in the same primary VLAN.
D
It can communicate with all hosts in primary VLAN 100
Why wrong: Private VLANs isolate host ports from each other.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It can communicate with the promiscuous port only
In a private VLAN (PVLAN) configuration, a host in a community or isolated secondary VLAN cannot communicate with other hosts in the same primary VLAN unless they share the same secondary VLAN. However, the question specifies that the host is in PVLAN 100 and its associated secondary PVLAN is 200, but does not state that the host is in a community VLAN; the only way a host in a secondary VLAN can communicate with any other host is via the promiscuous port (typically connected to a router or firewall). Since the host is in an isolated secondary VLAN (or community without a matching community), it can only send traffic to the promiscuous port, which then forwards it as needed.
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.
✗
It cannot communicate with any other host
Why it's wrong here
It can communicate with the promiscuous port.
✗
It can communicate only with other hosts in the same secondary VLAN 200
Why it's wrong here
Host ports in private VLANs cannot communicate with other host ports, regardless of secondary VLAN.
✓
It can communicate with the promiscuous port only
Why this is correct
Private VLAN host ports can only communicate with the promiscuous port in the same primary VLAN.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
It can communicate with all hosts in primary VLAN 100
Why it's wrong here
Private VLANs isolate host ports from each other.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the misconception that a host in a private VLAN can communicate with all other hosts in the same primary VLAN, but the correct behavior is that host-to-host communication is restricted to the same secondary VLAN (if community) or only to the promiscuous port (if isolated), and the question's phrasing 'associated secondary PVLAN is 200' implies an isolated VLAN unless stated otherwise.
Detailed technical explanation
How to think about this question
Private VLANs (PVLANs) use a primary VLAN (e.g., 100) and multiple secondary VLANs (isolated or community). An isolated secondary VLAN (e.g., 200) allows a host to communicate only with the promiscuous port, while a community secondary VLAN allows hosts within that community to communicate with each other and the promiscuous port. The switch enforces this by mapping the secondary VLAN to the primary VLAN and using port-based ACLs or VLAN access control lists (VACLs) to block traffic between isolated ports. In Cisco IOS, the command 'switchport private-vlan host-association 100 200' configures the port as a host port in secondary VLAN 200, and the promiscuous port is configured with 'switchport private-vlan mapping 100 200' to allow upstream routing.
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 security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: It can communicate with the promiscuous port only — In a private VLAN (PVLAN) configuration, a host in a community or isolated secondary VLAN cannot communicate with other hosts in the same primary VLAN unless they share the same secondary VLAN. However, the question specifies that the host is in PVLAN 100 and its associated secondary PVLAN is 200, but does not state that the host is in a community VLAN; the only way a host in a secondary VLAN can communicate with any other host is via the promiscuous port (typically connected to a router or firewall). Since the host is in an isolated secondary VLAN (or community without a matching community), it can only send traffic to the promiscuous port, which then forwards it as needed.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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