- 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.
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."
300-410 IPv6 Traffic Filtering and uRPF Practice Question
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.
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 uses TCP port 179. The ACL permits TCP port 179, which allows the BGP session to establish. However, BGP updates are sent over the same TCP connection, so they should be permitted. The issue may be that the ACL also needs to permit the BGP keepalives and notifications, but they use the same port. A more subtle edge case: if the ACL permits only TCP port 179, it may inadvertently block ICMPv6 packets needed for PMTUD or neighbor discovery, but the BGP session establishes. The most likely explanation: the ACL is applied inbound and the BGP updates are sent from a source port of 179 (or ephemeral), but the ACL might be matching only destination port 179. If the ACL permits only 'tcp any any eq 179', it matches packets with destination port 179. BGP updates are sent from the BGP router's source port 179 to the neighbor's ephemeral port, so the destination port is not 179. Thus, the updates are dropped.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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
OSPF neighbours must agree on key parameters.
- ✗
The ACL must also permit ICMPv6 for PMTUD, but the BGP session establishes, so PMTUD is not needed.
- ✗
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: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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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 — OSPF neighbours must agree on key parameters..
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 uses TCP port 179. The ACL permits TCP port 179, which allows the BGP session to establish. However, BGP updates are sent over the same TCP connection, so they should be permitted. The issue may be that the ACL also needs to permit the BGP keepalives and notifications, but they use the same port. A more subtle edge case: if the ACL permits only TCP port 179, it may inadvertently block ICMPv6 packets needed for PMTUD or neighbor discovery, but the BGP session establishes. The most likely explanation: the ACL is applied inbound and the BGP updates are sent from a source port of 179 (or ephemeral), but the ACL might be matching only destination port 179. If the ACL permits only 'tcp any any eq 179', it matches packets with destination port 179. BGP updates are sent from the BGP router's source port 179 to the neighbor's ephemeral port, so the destination port is not 179. Thus, the updates are dropped.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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