hardMultiple ChoiceObjective-mapped
300-410 Practice Question: A large enterprise network is experiencing…
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.
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.
- ✗
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.
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Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
Key term
BGP Prefix Filtering
BGP Prefix Filtering is the practice of controlling which network routes (prefixes) a router accepts or advertises to its BGP neighbors, preventing unwanted or harmful routes from spreading across the internet.
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