The answer is that the ACL is blocking TCP port 179 packets, but keepalives are permitted due to a separate permit statement. This occurs because BGP relies on TCP port 179 for its session transport, including both update messages carrying Network Layer Reachability Information (NLRI) and keepalive packets. If the inbound ACL explicitly permits only specific prefixes or a generic permit statement for keepalives, the TCP segments containing BGP updates are dropped, preventing route installation, while the keepalives maintain the session state. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that ACLs filter at Layer 4, not BGP prefix-level, and that a session can stay up even when updates are blocked—a common trap where engineers assume a down session means blocked updates. The key memory tip is: “Keepalives keep the session, updates install the routes; if the ACL blocks TCP 179 data, you get a silent black hole.”
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
A network engineer configures an inbound IPv4 ACL on a router's interface to filter BGP updates from an eBGP neighbor. The ACL permits only specific prefixes. After applying the ACL, the BGP session remains established, but the router does not install any routes from that neighbor. What is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The ACL is blocking TCP port 179 packets, but keepalives are permitted due to a separate permit statement.
If the ACL permits keepalives (e.g., by allowing established connections) but denies the initial update packets, the session stays up but updates are filtered.
B
The ACL is applied outbound instead of inbound, filtering the router's own updates.
Why wrong: Outbound ACL would affect outgoing updates, not incoming.
C
The BGP neighbor has a distribute-list that is filtering routes.
Why wrong: Distribute-list is a BGP feature, not an interface ACL, and would be configured on the neighbor.
D
The ACL is using the wrong protocol number; BGP uses UDP port 179.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL is blocking TCP port 179 packets, but keepalives are permitted due to a separate permit statement.
The ACL is applied inbound on the router's interface to filter BGP updates from the eBGP neighbor. If the ACL permits only specific prefixes but does not explicitly permit TCP port 179 (BGP's transport protocol), the TCP packets carrying BGP updates and keepalives may be blocked. However, the BGP session remains established because keepalives are permitted by a separate permit statement (or because the session was established before the ACL was applied and the TCP connection is not reset). The router does not install routes because the ACL drops the BGP update messages (which contain the NLRI) while allowing keepalives to maintain the session.
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 ACL is blocking TCP port 179 packets, but keepalives are permitted due to a separate permit statement.
Why this is correct
If the ACL permits keepalives (e.g., by allowing established connections) but denies the initial update packets, the session stays up but updates are filtered.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL is applied outbound instead of inbound, filtering the router's own updates.
Why it's wrong here
Outbound ACL would affect outgoing updates, not incoming.
✗
The BGP neighbor has a distribute-list that is filtering routes.
Why it's wrong here
Distribute-list is a BGP feature, not an interface ACL, and would be configured on the neighbor.
✗
The ACL is using the wrong protocol number; BGP uses UDP port 179.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between filtering the BGP session (TCP port 179) and filtering the routing updates (NLRI) within the session; the trap here is that candidates assume an ACL that permits specific prefixes will automatically allow the BGP session to function, but the ACL must also permit the TCP transport for BGP to exchange updates.
Detailed technical explanation
How to think about this question
BGP relies on TCP for reliable transport; the TCP connection carries both keepalive messages (sent every 60 seconds by default) and update messages. An inbound ACL that permits only specific prefixes (e.g., using permit ip any any after the prefix entries) might still block TCP port 179 if the ACL does not explicitly permit TCP traffic on that port. A common real-world scenario is when an ACL is designed to filter routing updates but inadvertently blocks the underlying TCP session, causing the BGP session to flap or, as in this case, remain up but not pass updates.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL is blocking TCP port 179 packets, but keepalives are permitted due to a separate permit statement. — The ACL is applied inbound on the router's interface to filter BGP updates from the eBGP neighbor. If the ACL permits only specific prefixes but does not explicitly permit TCP port 179 (BGP's transport protocol), the TCP packets carrying BGP updates and keepalives may be blocked. However, the BGP session remains established because keepalives are permitted by a separate permit statement (or because the session was established before the ACL was applied and the TCP connection is not reset). The router does not install routes because the ACL drops the BGP update messages (which contain the NLRI) while allowing keepalives to maintain the session.
What should I do if I get this 300-410 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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