- A
Only routes with prefix 10.0.0.0/8 and mask length between 16 and 24 are sent to neighbor with community 100:100.
Correct. The prefix-list permits 10.0.0.0/8 ge 16 le 24; matching routes get community set and are advertised.
- B
All routes with prefix 10.0.0.0/8 are sent to neighbor; routes with mask length between 16 and 24 get community 100:100.
Why wrong: Incorrect. The prefix-list only permits routes with mask length 16-24; 10.0.0.0/8 itself is not permitted.
- C
Routes that match the prefix-list are sent with community 100:100; all other routes are sent without any community.
Why wrong: Incorrect. Routes not matching the prefix-list are denied by the route-map because there is no catch-all permit.
- D
The configuration is invalid because the prefix-list uses ge and le together; only one can be used.
Why wrong: Incorrect. Using both ge and le is valid; it specifies a range of prefix lengths.
Quick Answer
The correct answer is that only routes matching 10.0.0.0/8 with a mask length between 16 and 24 are sent to the BGP neighbor with community 100:100. This is because the prefix-list PL-2 uses the ge (greater-or-equal) and le (less-or-equal) operators to filter based on subnet mask length, not just the network prefix; here, ge 16 le 24 means the prefix must have a mask of at least 16 bits and at most 24 bits, so 10.0.0.0/8 itself is excluded while 10.1.0.0/16 or 10.1.1.0/24 are permitted. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how prefix-list ge/le interact with BGP route-map filtering—a common trap is forgetting that the base prefix (10.0.0.0/8) does not automatically match unless its own mask falls within the ge/le range. Memory tip: think of ge/le as a "mask-length gate"—the prefix must first match the network, then pass through the mask-length window.
300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. 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.
Consider the following configuration on R5:
!--- R5 configuration
ip prefix-list PL-2 seq 5 permit 10.0.0.0/8 ge 16 le 24
! route-map RMAP permit 10 match ip address prefix-list PL-2 set community 100:100 !
router bgp 65200 neighbor 192.168.1.2 route-map RMAP out
!
What is the effect of this configuration?
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
Only routes with prefix 10.0.0.0/8 and mask length between 16 and 24 are sent to neighbor with community 100:100.
The prefix-list PL-2 permits prefixes that match 10.0.0.0/8 with a mask length greater than or equal to 16 and less than or equal to 24. So, for example, 10.1.0.0/16, 10.1.1.0/24 are permitted, but 10.0.0.0/8 itself (mask 8) is not. The route-map RMAP sets community 100:100 on matching routes. Routes that do not match the prefix-list are not processed by this route-map (since there is only one sequence), so they are denied (not sent to neighbor).
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.
- ✓
Only routes with prefix 10.0.0.0/8 and mask length between 16 and 24 are sent to neighbor with community 100:100.
Why this is correct
Correct. The prefix-list permits 10.0.0.0/8 ge 16 le 24; matching routes get community set and are advertised.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
All routes with prefix 10.0.0.0/8 are sent to neighbor; routes with mask length between 16 and 24 get community 100:100.
Why it's wrong here
Incorrect. The prefix-list only permits routes with mask length 16-24; 10.0.0.0/8 itself is not permitted.
- ✗
Routes that match the prefix-list are sent with community 100:100; all other routes are sent without any community.
Why it's wrong here
Incorrect. Routes not matching the prefix-list are denied by the route-map because there is no catch-all permit.
- ✗
The configuration is invalid because the prefix-list uses ge and le together; only one can be used.
Why it's wrong here
Incorrect. Using both ge and le is valid; it specifies a range of prefix lengths.
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?
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: Only routes with prefix 10.0.0.0/8 and mask length between 16 and 24 are sent to neighbor with community 100:100. — The prefix-list PL-2 permits prefixes that match 10.0.0.0/8 with a mask length greater than or equal to 16 and less than or equal to 24. So, for example, 10.1.0.0/16, 10.1.1.0/24 are permitted, but 10.0.0.0/8 itself (mask 8) is not. The route-map RMAP sets community 100:100 on matching routes. Routes that do not match the prefix-list are not processed by this route-map (since there is only one sequence), so they are denied (not sent to neighbor).
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
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Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Consider the following configuration on router R2: !--- R2 configuration ip prefix-list FILTER seq 5 deny 10.1.0.0/16 le 24 ip prefix-list FILTER seq 10 permit 0.0.0.0/0 le 32 ! route-map BGP-IN permit 10 match ip address prefix-list FILTER ! router bgp 65000 neighbor 192.168.1.1 route-map BGP-IN in ! What is the effect of this configuration?
medium- A.All routes from neighbor 192.168.1.1 are accepted; the prefix-list is not applied correctly because the route-map only has a permit sequence.
- ✓ B.Routes within 10.1.0.0/16 with mask length 24 or shorter are denied; all other routes are permitted.
- C.Only routes with mask length exactly 24 are denied; all other routes are permitted.
- D.The configuration is incomplete; a route-map must have a deny statement to filter routes.
Why B: The prefix-list FILTER denies any prefix within 10.1.0.0/16 with a mask length less than or equal to 24 (i.e., 10.1.0.0/16 through 10.1.255.0/24). The permit statement allows all other prefixes. The route-map BGP-IN calls this prefix-list; since there is only one permit sequence, routes that match the deny statement in the prefix-list are implicitly denied by the route-map. Therefore, routes like 10.1.0.0/16, 10.1.1.0/24, etc., are filtered out.
Last reviewed: Jun 18, 2026
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