The answer is that the AS-path is prepended with the local AS before the inbound route map is applied, so the regex does not match the original AS-path. When a router receives an eBGP update, it automatically adds its own AS number to the beginning of the AS-path sequence before any inbound route-map processing occurs. This means that if you configure eBGP inbound route-map AS-path filtering to match the neighbor’s AS using a regex, the match will fail because the neighbor’s AS is no longer the first element—it has been pushed to the second position by the local AS prepend. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap: engineers forget that AS-path modification happens prior to route-map evaluation, leading to unexpected route acceptance. The key takeaway is that inbound filtering sees the path after the local AS is added, not the path as advertised. Memory tip: think "local first, filter second"—the router always stamps its own AS onto the path before checking your regex.
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.
An engineer configures a route map to filter BGP routes based on AS-path using a regex. The route map is applied inbound to an eBGP neighbor. The engineer notices that routes with an AS-path containing the neighbor's AS are still being accepted. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The AS-path is prepended with the local AS before the inbound route map is applied, so the regex does not match the original AS-path.
When an eBGP route is received, the router prepends its own AS to the AS-path before the route map is applied. Therefore, if the route map is trying to match the neighbor's AS in the AS-path, it will fail because the neighbor's AS is now the first AS in the path. The edge case is that the AS-path is modified before the route map is processed.
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 AS-path is prepended with the local AS before the inbound route map is applied, so the regex does not match the original AS-path.
Why this is correct
eBGP prepends the local AS before route map processing; the route map sees the modified path.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route map is applied outbound instead of inbound.
Why it's wrong here
Outbound would affect routes sent, not received.
✗
The regex is incorrect; it should use _AS_ to match the AS number.
Why it's wrong here
Even with correct regex, the AS is prepended, so the neighbor's AS is not the first.
✗
The neighbor is configured with 'send-community' which overrides the route map.
Why it's wrong here
Send-community does not affect AS-path filtering.
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.
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 AS-path is prepended with the local AS before the inbound route map is applied, so the regex does not match the original AS-path. — When an eBGP route is received, the router prepends its own AS to the AS-path before the route map is applied. Therefore, if the route map is trying to match the neighbor's AS in the AS-path, it will fail because the neighbor's AS is now the first AS in the path. The edge case is that the AS-path is modified before the route map is processed.
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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