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.
A multinational corporation operates a private MPLS VPN network connecting 50 branch offices to a central data center. The network uses BGP as the routing protocol within the VPN, with each branch announcing its internal prefixes to the data center routers. Over the past week, several branch offices have reported intermittent connectivity issues, with traffic being routed to incorrect destinations before recovering. Network logs show that during these incidents, the data center router receives unexpected BGP updates from one of the branch routers, advertising prefixes that belong to other branches. BGP sessions remain established without flaps. The security team is concerned that this could be a route leak or intentional hijack. The network engineer has verified that all BGP sessions are authenticated with MD5 and that RPKI validation is not currently deployed. Which course of action should the engineer take first to mitigate the issue?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure inbound BGP prefix filtering on the data center routers.
The intermittent connectivity issues are caused by a branch router advertising prefixes that belong to other branches, which is a classic route leak or hijack scenario. Configuring inbound BGP prefix filtering on the data center routers is the immediate and most effective mitigation because it allows the engineer to explicitly define which prefixes are accepted from each BGP neighbor, preventing unauthorized or incorrect routes from being installed in the routing table. This approach does not require additional infrastructure or protocol changes and directly addresses the root cause of the traffic misdirection.
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.
✓
Configure inbound BGP prefix filtering on the data center routers.
Why this is correct
Filters out unauthorized prefixes from branch routers, stopping the leak.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Implement BGP Flowspec to rate-limit traffic to the affected prefixes.
Why it's wrong here
Flowspec is for traffic filtering, not for preventing route leaks.
✗
Deploy RPKI validation across all routers.
Why it's wrong here
RPKI validates origin but requires setup; not an immediate fix.
✗
Increase the BGP hold timer on data center routers.
Why it's wrong here
Hold timer affects session keepalive, not route advertisement content.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume RPKI is the best first step because it is a modern security mechanism, but the question explicitly states it is not deployed and asks for the first action to mitigate the issue, making immediate inbound filtering the correct answer over a longer-term deployment.
Detailed technical explanation
How to think about this question
Inbound BGP prefix filtering can be implemented using prefix lists, route maps, or AS-path access lists to match and permit only the specific prefixes that a branch is authorized to advertise. For example, a data center router can be configured with a prefix list that permits only the /24 subnets assigned to that branch, and denies all others, effectively creating a whitelist. This technique is recommended in RFC 7454 (BGP Operations and Security) as a fundamental BGP security best practice, and it is often combined with maximum prefix limits to protect against accidental or malicious route leaks.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
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: Configure inbound BGP prefix filtering on the data center routers. — The intermittent connectivity issues are caused by a branch router advertising prefixes that belong to other branches, which is a classic route leak or hijack scenario. Configuring inbound BGP prefix filtering on the data center routers is the immediate and most effective mitigation because it allows the engineer to explicitly define which prefixes are accepted from each BGP neighbor, preventing unauthorized or incorrect routes from being installed in the routing table. This approach does not require additional infrastructure or protocol changes and directly addresses the root cause of the traffic misdirection.
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: "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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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