JN0-106 Routing Fundamentals Practice Question
Exhibit
show route protocol bgp
inet.0: 15 destinations, 20 routes (14 active, 0 holddown, 1 hidden)
+ = Active Route, - = Last Active, * = Both
172.16.0.0/12 *[BGP/170] 00:01:20, MED 0, localpref 100, from 192.168.1.1
AS path: 64500 65000 I
> to 10.0.0.2 via ge-0/0/0.0
10.0.0.0/8 *[BGP/170] 00:02:00, MED 0, localpref 200, from 192.168.1.2
AS path: 64500 I
> to 10.0.0.1 via ge-0/0/0.0
192.168.0.0/16 [BGP/170] 00:01:00, MED 0, localpref 100, from 192.168.1.1
AS path: 64500 65000 I
(hidden)Refer to the exhibit. The route for 192.168.0.0/16 is hidden. What is the most likely reason?
⚠ Common exam trap
Test-takers frequently confuse route selection attributes (like local preference, AS path, MED) with route installation conditions, assuming a less preferred attribute hides the route, when in fact only next-hop unreachability or policy-based rejection can cause a route to be hidden in JUNOS.
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 BGP next-hop is unreachable
In JUNOS, a BGP route is hidden when the next-hop address is not reachable via any active route in the routing table. The router cannot install the route into the forwarding table because it lacks a valid path to the next-hop, causing the route to be marked as hidden. This is a common issue when the next-hop is not covered by a directly connected or static route, or when the IGP route to the next-hop is missing.
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 BGP next-hop is unreachable
Why this is correct
In Junos, a BGP route is considered 'hidden' when it cannot be installed into the inet.0 routing table because its BGP next-hop address is not resolvable by an active IPv4 or IPv6 route. The route remains in the BGP database (Adj-RIB-In) but is excluded from forwarding and from best-path selection. Use 'show route protocol bgp hidden' to display such routes; as soon as next-hop reachability is restored, the route becomes visible and active.
- ✗
The route has a lower local preference
Why it's wrong here
Local preference is a BGP path attribute used during best-path selection to choose the preferred egress route, but it does not affect a route's validity or eligibility for installation. Even if this route had a lower local preference than an alternative path, BGP would still consider it a valid candidate; it would simply lose the best-path decision. A lower local preference does not cause a route to be hidden—hidden status is reserved for routes that fail structural requirements such as next-hop resolution or import policy rejection, not for routes that score lower in an attribute comparison.
- ✗
The route has a longer AS path
Why it's wrong here
AS path length is a criterion in the BGP best-path algorithm that prefers shorter paths, but it is irrelevant to whether a route can be accepted into the routing table. A longer AS path makes this route less preferred, meaning it will not be selected as active if a shorter-path route exists; however, the route is still a full-fledged BGP route and remains visible in the BGP RIB. In fact, if this were the only route for the prefix, it would be installed and active. Hidden routes are caused by fundamental acceptance failures, such as an unreachable next-hop, not by being outscored in path selection.
- ✗
The route has a higher MED value
Why it's wrong here
MED (Multi-Exit Discriminator) is a metric exchanged between autonomous systems to influence inbound traffic when multiple entry points exist into the same AS, but it does not determine route eligibility. In this exhibit, the MED is 0, identical to other candidate routes, so MED provides no differentiation at all. Even a higher MED would merely cause the route to lose a best-path comparison among routes from the same neighboring AS; the route would still be present and usable. A route is only hidden if it cannot be resolved or otherwise fails acceptance criteria—never solely because of a high MED value.
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