Question 1,615 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

Why BGP Routes Are Not Exchanged Despite ACL Permitting TCP 179

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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.

An engineer configures an IPv6 ACL to permit BGP traffic (TCP port 179) between two routers and deny all other traffic. The ACL is applied inbound on the interface facing the BGP neighbor. BGP session establishes, but the routers cannot exchange IPv6 routes. Which 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.

A
The ACL permits only TCP packets with destination port 179, but BGP updates are sent from source port 179 to an ephemeral destination port, so they are not matched and are dropped.
Correct. BGP uses source port 179 for outgoing updates; the destination port is ephemeral. The ACL must permit both directions or use 'tcp any any eq 179' for incoming updates, but for outgoing updates, the router needs to permit 'tcp any any' or specify the correct direction.
B
The ACL must also permit ICMPv6 for PMTUD, but the BGP session establishes, so PMTUD is not needed.
Why wrong: Incorrect. BGP can establish without PMTUD if the MTU is sufficient.
C
The ACL is applied outbound, not inbound, causing the BGP updates to be filtered.
Why wrong: Incorrect. The question states the ACL is applied inbound.
D
The router has BGP authentication configured, which changes the TCP port number.
Why wrong: Incorrect. Authentication does not change the port number.

Full breakdown with real-world context →

Quick Answer

The answer is that the ACL permits only TCP packets with destination port 179, but BGP updates are sent from source port 179 to an ephemeral destination port, so they are not matched and are dropped. This occurs because the ACL statement `permit tcp any any eq 179` only matches traffic destined for port 179, while BGP route exchange uses the established TCP connection where updates flow from the source port 179 of the advertising router to a random high-numbered destination port on the neighbor. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of TCP port directionality in stateful protocols—a common trap is assuming a single permit for port 179 covers all BGP traffic, when in fact the session establishes but route updates are silently discarded. Remember the memory tip: "BGP speaks from 179, but listens on 179—your ACL must permit both directions or use 'established' to allow return traffic."

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The ACL permits only TCP packets with destination port 179, but BGP updates are sent from source port 179 to an ephemeral destination port, so they are not matched and are dropped.

BGP updates are sent from TCP source port 179 to a high ephemeral destination port (above 1023). The ACL only permits TCP packets with destination port 179, which matches the initial BGP session establishment (where the neighbor sends SYN to destination port 179). However, once the session is up, BGP update messages are sourced from port 179 and destined to the ephemeral port of the peer. These updates have a destination port that is not 179, so they are not permitted by the ACL and are dropped, preventing route exchange.

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 permits only TCP packets with destination port 179, but BGP updates are sent from source port 179 to an ephemeral destination port, so they are not matched and are dropped.
Why this is correct
Correct. BGP uses source port 179 for outgoing updates; the destination port is ephemeral. The ACL must permit both directions or use 'tcp any any eq 179' for incoming updates, but for outgoing updates, the router needs to permit 'tcp any any' or specify the correct direction.
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 must also permit ICMPv6 for PMTUD, but the BGP session establishes, so PMTUD is not needed.
Why it's wrong here
Incorrect. BGP can establish without PMTUD if the MTU is sufficient.
✗
The ACL is applied outbound, not inbound, causing the BGP updates to be filtered.
Why it's wrong here
Incorrect. The question states the ACL is applied inbound.
✗
The router has BGP authentication configured, which changes the TCP port number.
Why it's wrong here
Incorrect. Authentication does not change the port number.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that BGP traffic always uses destination port 179, but the trap is that BGP updates are sent from source port 179 to an ephemeral destination port, so an ACL permitting only destination port 179 will block route updates while still allowing the session to establish.

Detailed technical explanation

How to think about this question

BGP uses TCP port 179 for both source and destination during session establishment, but after the TCP three-way handshake, data packets (including OPEN, KEEPALIVE, UPDATE, and NOTIFICATION) are sent from port 179 to the ephemeral port of the peer. This asymmetric port behavior is defined in RFC 4271. When filtering inbound, an ACL must permit both directions of traffic: TCP from any source port to destination port 179 (for incoming session setup) and TCP from source port 179 to any destination port (for incoming updates). A common real-world mistake is to only permit destination port 179, which breaks route exchange.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The ACL permits only TCP packets with destination port 179, but BGP updates are sent from source port 179 to an ephemeral destination port, so they are not matched and are dropped. — BGP updates are sent from TCP source port 179 to a high ephemeral destination port (above 1023). The ACL only permits TCP packets with destination port 179, which matches the initial BGP session establishment (where the neighbor sends SYN to destination port 179). However, once the session is up, BGP update messages are sourced from port 179 and destined to the ephemeral port of the peer. These updates have a destination port that is not 179, so they are not permitted by the ACL and are dropped, preventing route exchange.

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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