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300-410 Practice Question: Is troubleshooting an OSPF network where an ABR…
A network engineer is troubleshooting an OSPF network where an ABR (R1) is configured with the 'area 1 range 10.0.0.0 255.255.0.0' command to summarize routes from area 1 into area 0. After the configuration, routers in area 0 lose connectivity to the 10.0.1.0/24 subnet, although the summary route 10.0.0.0/16 is present in their routing tables. 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 summary route is installed, but the next-hop IP address for the summary route is not reachable from routers in area 0, causing traffic to be dropped.
The issue is that the summary route may be installed, but the more specific routes are suppressed. If the summary route points to a next hop that is not reachable or has a higher metric, traffic may be black-holed.
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 summary route 10.0.0.0/16 is being advertised with a metric of 0, causing routers to prefer a default route instead.
Why it's wrong here
Incorrect. A metric of 0 is valid and would not cause loss of connectivity to a specific subnet.
- ✗
The ABR is not generating the summary route because the component routes are not all in the same area.
Why it's wrong here
Incorrect. The area range command summarizes routes within the specified area.
- ✗
The 10.0.1.0/24 subnet is not included in the summary range because the range command uses a network mask that does not match the subnet's network address.
Why it's wrong here
Incorrect. 10.0.1.0/24 is within the 10.0.0.0/16 range.
- ✓
The summary route is installed, but the next-hop IP address for the summary route is not reachable from routers in area 0, causing traffic to be dropped.
Why this is correct
Correct. In OSPF, the summary route's next hop is set to the ABR's interface IP. If that interface is down or the path is not reachable, traffic to the summary may fail, and since specific routes are suppressed, connectivity to the subnet is lost.
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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