Floating Static Route: Preference-Based Backup
An engineer wants to configure a static route that will be used only if the primary route (learned via OSPF) becomes unavailable. Which feature should be used?
⚠ Common exam trap
A common mix-up: candidates confuse 'metric' with 'preference' (administrative distance), assuming a lower metric on a static route would make it preferred over OSPF, but metric only applies within the same routing protocol, not between different sources.
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
✓
Floating static route with a higher preference.
A floating static route is configured with a higher preference (administrative distance) than the OSPF-learned route. Since JUNOS prefers routes with lower preference values, the static route will only be installed in the routing table when the OSPF route is withdrawn, providing a backup path.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Floating static route with a higher preference.
Why this is correct
A floating static route is configured with a higher preference value than the dynamic protocol's default. Since OSPF's default preference is 10, setting the static route's preference to, say, 15 ensures it is only used when the OSPF route is unavailable. This makes it a backup route.
- ✗
Static route with a lower metric.
Why it's wrong here
In Junos, route preference is the primary criterion for selecting between routes from different routing protocols, while metric is only compared among routes from the same protocol. A lower metric on a static route does not help it win against an OSPF route because the OSPF route's preference (10) is already more favorable than the static route's default preference (5). Thus, merely lowering the metric has no effect on cross-protocol selection.
- ✗
Reverse path forwarding.
Why it's wrong here
Reverse Path Forwarding (RPF) is a unicast packet-forwarding security check that verifies the incoming packet's source address has a valid reverse route in the FIB. It does not influence route selection or create redundancy. If the primary route fails, RPF might actually drop traffic for the affected source until the floating static route is installed.
- ✗
Routing policy.
Why it's wrong here
Routing policies in Junos can filter, modify, or set attributes like preference when routes are imported into the routing table. However, a routing policy is not the specific mechanism for creating a backup static route; it would require additional configuration to manually adjust preference. The direct configuration of a higher preference on the static route itself is the standard way to implement a floating static route.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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