- A
The route-map RMAP-IN is missing a permit statement for other prefixes, causing implicit deny of all routes except the default.
The route-map has only one permit sequence matching the default route, so all other VPNv4 routes are denied by the implicit deny at the end of the route-map. This prevents R1 from receiving routes for other prefixes.
- B
The prefix-list PL-ALLOW is misconfigured and does not match the default route correctly.
Why wrong: The show output indicates the default route is received, so the prefix-list is matching correctly for that route.
- C
The VRF CUSTOMER on R1 is missing the route-target import 100:1 command.
Why wrong: The route-map sets the RT to 100:1, so if the VRF imports RT 100:1, the route should be installed. The issue is that other routes are not received at all.
- D
The BGP session between R1 and R2 is flapping due to a mismatch in the BGP update-source.
Why wrong: The show output shows that routes are being received (the default route), so the BGP session is stable.
Quick Answer
The answer is the route-map implicit deny in MPLS L3VPN environments, where a missing permit statement causes routes to be silently dropped. This occurs because route-maps in Cisco IOS have an implicit deny at the end, meaning any prefix not explicitly matched by a permit clause is rejected. In the given scenario, the route-map RMAP-IN only permits the default route via prefix-list PL-ALLOW, so all other VPNv4 routes from R2 are denied before they can be installed into the VRF, explaining why only the default appears in received-routes but not in the VRF table. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how route-maps interact with BGP prefix filtering and extended community manipulation—a common trap is assuming a permit statement alone is sufficient without considering the implicit deny for unmatched prefixes. Remember the mnemonic: "If you permit one, you deny the rest—unless you add a permit any."
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 large enterprise network is experiencing intermittent connectivity between Site A (R1) and Site B (R2) over an MPLS L3VPN. R1 has the following relevant configuration: route-map RMAP-IN permit 10 match ip address prefix-list PL-ALLOW set extcommunity rt 100:1. Router R2 shows: 'show bgp vpnv4 unicast all neighbors 10.1.1.1 received-routes' lists only the default route, but 'show ip route vrf CUSTOMER' shows no default route. What is the root cause?
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 route-map RMAP-IN is missing a permit statement for other prefixes, causing implicit deny of all routes except the default.
The route-map RMAP-IN on R1 is configured to set the route-target extended community (RT) on incoming BGP VPNv4 routes. However, the route-map is applied inbound on the BGP session, which means it modifies routes received from R2. The match condition uses prefix-list PL-ALLOW, which likely permits only the default route. The set extcommunity rt 100:1 command overwrites or adds the RT, but if the route-map does not explicitly permit other routes (e.g., via a subsequent permit statement), those routes are denied by the implicit deny at the end of the route-map. This causes R1 to reject all routes except the default, but the default route may not be installed in the VRF due to missing RT matching or other VRF configuration issues. The correct fix is to add a permit statement for all necessary prefixes or remove the route-map from the neighbor statement.
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 route-map RMAP-IN is missing a permit statement for other prefixes, causing implicit deny of all routes except the default.
Why this is correct
The route-map has only one permit sequence matching the default route, so all other VPNv4 routes are denied by the implicit deny at the end of the route-map. This prevents R1 from receiving routes for other prefixes.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The prefix-list PL-ALLOW is misconfigured and does not match the default route correctly.
Why it's wrong here
The show output indicates the default route is received, so the prefix-list is matching correctly for that route.
- ✗
The VRF CUSTOMER on R1 is missing the route-target import 100:1 command.
Why it's wrong here
The route-map sets the RT to 100:1, so if the VRF imports RT 100:1, the route should be installed. The issue is that other routes are not received at all.
- ✗
The BGP session between R1 and R2 is flapping due to a mismatch in the BGP update-source.
Why it's wrong here
The show output shows that routes are being received (the default route), so the BGP 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 show output indicates the default route is received, so the prefix-list is matching correctly for that route.
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: The route-map RMAP-IN is missing a permit statement for other prefixes, causing implicit deny of all routes except the default. — The route-map RMAP-IN on R1 is configured to set the route-target extended community (RT) on incoming BGP VPNv4 routes. However, the route-map is applied inbound on the BGP session, which means it modifies routes received from R2. The match condition uses prefix-list PL-ALLOW, which likely permits only the default route. The set extcommunity rt 100:1 command overwrites or adds the RT, but if the route-map does not explicitly permit other routes (e.g., via a subsequent permit statement), those routes are denied by the implicit deny at the end of the route-map. This causes R1 to reject all routes except the default, but the default route may not be installed in the VRF due to missing RT matching or other VRF configuration issues. The correct fix is to add a permit statement for all necessary prefixes or remove the route-map from the neighbor statement.
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.
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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. What is the default behavior of a route-map when a route does not match any match clause in any sequence?
easy- A.The route is permitted by default.
- ✓ B.The route is denied by default.
- C.The route is processed by the last sequence regardless of match.
- D.The route is forwarded to the next route-map if one exists.
Why B: A route-map consists of sequences with permit or deny actions. If a route does not match any match clause in any sequence, it is implicitly denied. This is similar to an access-list: there is an implicit deny at the end of the route-map.
Last reviewed: Jun 18, 2026
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