JN0-106 Routing Fundamentals Practice Question
You are configuring a Juniper MX router to act as a BGP route reflector for your ISP network. The router has several iBGP peers, including clients and non-clients. You have configured the route-reflector-cluster-id and set the clients. After the configuration, you notice that some prefixes are not being reflected to a specific client router. The client has a valid BGP session to the route reflector and can see other prefixes. You check the BGP routing table on the route reflector and see that the missing prefixes are present but have the 'non-routable' flag. The route reflector's BGP table shows the prefix with a next-hop that is not reachable via an IGP route. What is the most likely 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 next-hop for the missing prefixes is not reachable via the IGP on the route reflector; the route reflector needs a route to the next-hop to advertise the prefix.
The correct answer is A: the next-hop for the missing prefixes is not reachable via the IGP on the route reflector, and the route reflector needs a route to the next-hop to advertise the prefix. In Junos, a BGP route is marked non-routable (hidden) when the next-hop cannot be resolved in the inet.0 routing table, and such unresolved routes are not eligible for advertisement to BGP peers, including route-reflector clients. Since the session is up and other prefixes are received, the issue is next-hop resolution rather than session or reflection configuration. Option B is wrong because an incorrect cluster ID would cause loop-prevention behavior (such as rejecting routes with its own cluster ID), not a non-routable flag from an unresolved next-hop. Option C is wrong because ORIGINATOR_ID suppression would drop routes originated by the same router, not mark them non-routable. Option D is wrong because next-hop-self changes the advertised next-hop to the route reflector's own address, which is locally reachable and would not cause this non-routable condition.
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 next-hop for the missing prefixes is not reachable via the IGP on the route reflector; the route reflector needs a route to the next-hop to advertise the prefix.
Why this is correct
For a BGP route to be eligible for advertisement, the router must be able to resolve the next-hop through an active route in its routing table. On a route reflector, this means the IGP (such as OSPF or IS-IS) must have a route to the next-hop IP of the reflected prefix. If that route is missing, the prefix is flagged as non-routable and is not advertised to clients, exactly as observed in the question.
- ✗
The route-reflector-cluster-id is set incorrectly, causing the route reflector to drop the route to prevent loops.
Why it's wrong here
The route-reflector-cluster-id is a mechanism to prevent loops in a route-reflector topology. A route reflector ignores updates that carry its own cluster-id, as that indicates the route has already been reflected by this cluster. An incorrect cluster-id would not cause a prefix to be flagged as non-routable; it would either fail to prevent loops or incorrectly reject some updates, but it does not affect next-hop reachability or route validity in the routing table.
- ✗
The ORIGINATOR_ID attribute from a previous route reflector is causing the route to be suppressed.
Why it's wrong here
The ORIGINATOR_ID attribute is used to prevent loopbacks; a BGP router ignores a route if the ORIGINATOR_ID matches its own router ID. This attribute is set by the original route reflector that first reflected the route, and it is passed along in the path. However, suppressing a route due to ORIGINATOR_ID would result in the route being ignored completely, not flagged as 'non-routable'. The 'non-routable' flag specifically arises from unresolved next-hop, not from loop-prevention mechanisms.
- ✗
The route reflector is using the next-hop-self option and cannot resolve its own IP as a next-hop.
Why it's wrong here
Next-hop-self is a configuration option on a route reflector that changes the next-hop attribute to the route reflector's own IP address for routes advertised to clients. Since the route reflector has a local interface with that IP, the next-hop is inherently reachable, so the route would be routable. In this scenario, the missing prefixes are likely due to the original next-hop being unreachable via the IGP, not because the route reflector cannot resolve its own IP.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JN0-106 exam.