The answer is that the prefix lists FILTER_IN and FILTER_OUT are allowing only 2 prefixes in each direction. This is confirmed by the show ip bgp neighbors output, which displays the applied inbound and outbound prefix filters under the IPv4 Unicast address family section, alongside the "Prefixes Sent" and "Prefixes Rcvd" counters both showing a value of 2. The BGP prefix-list filtering verification with the show command directly reveals the filtering effect by comparing the number of prefixes exchanged against the configured filter lists, meaning any mismatch between the filter rules and the prefix count indicates a filtering action. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to interpret the show ip bgp neighbors output to verify prefix-list application and effectiveness, with a common trap being to overlook the "Incoming update prefix filter list" and "Outgoing update prefix filter list" lines or to confuse the update count with the prefix count. A useful memory tip is "Two lists, two prefixes" — when you see both FILTER_IN and FILTER_OUT applied, always check the Prefixes Sent and Rcvd fields to confirm the exact number of routes being permitted.
300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the following command on Router R1:
R1# show ip bgp neighbors 192.168.1.2
BGP neighbor is 192.168.1.2, remote AS 65001, external link
BGP version 4, remote router ID 10.2.2.2
BGP state = Established, up for 00:15:00
Last read 00:00:05, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is multisession capable
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh: advertised
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 10 5
Keepalives: 15 15
Route Refresh: 0 0
Total: 26 21
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
Session: 192.168.1.2
BGP table version 14, neighbor version 14/0
Output queue size : 0
Index 1, Advertise bit 0
1 update-group member
Incoming update prefix filter list: FILTER_IN
Outgoing update prefix filter list: FILTER_OUT
Sent Rcvd
Prefixes: 2 2
Updates: 10 5
Based on this output, what is the effect of the prefix lists?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The prefix lists FILTER_IN and FILTER_OUT are allowing only 2 prefixes in each direction.
The output shows that incoming and outgoing prefix filters (FILTER_IN and FILTER_OUT) are applied. The prefixes sent and received are both 2, indicating that the filters are allowing exactly 2 prefixes in each direction. The correct answer is that the prefix lists are filtering routes to only 2 prefixes.
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 prefix lists FILTER_IN and FILTER_OUT are allowing only 2 prefixes in each direction.
Why this is correct
The counters show 2 prefixes sent and received, matching the filter lists.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
The prefix lists are not applied because the session is established.
Why it's wrong here
The output shows they are applied.
✗
The prefix lists are misconfigured and causing route flapping.
Why it's wrong here
No evidence of flapping; session is stable.
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.
Trap categories for this question
Command / output trap
The output shows they are applied.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The prefix lists FILTER_IN and FILTER_OUT are allowing only 2 prefixes in each direction. — The output shows that incoming and outgoing prefix filters (FILTER_IN and FILTER_OUT) are applied. The prefixes sent and received are both 2, indicating that the filters are allowing exactly 2 prefixes in each direction. The correct answer is that the prefix lists are filtering routes to only 2 prefixes.
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: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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