hardMultiple Choice
350-401 Practice Question: Is troubleshooting an OSPF issue where a router…
A network engineer is troubleshooting an OSPF issue where a router is not learning a route to a network that is advertised via a type 5 LSA from an ASBR. The engineer checks the OSPF database and sees the type 5 LSA, but the route is not in the routing table. The forwarding address in the LSA is 0.0.0.0. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that a Type 5 LSA present in the database automatically guarantees the route is installed, but the trap here is that the forwarding address of 0.0.0.0 requires the ASBR to be reachable via an OSPF internal route, which is a common oversight.
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 ASBR is not reachable via an OSPF internal route.
When a Type 5 LSA has a forwarding address of 0.0.0.0, OSPF routers will use the ASBR as the next hop for the external route. For the route to be installed in the routing table, the ASBR must be reachable via an OSPF intra-area or inter-area route. If the ASBR is not reachable (e.g., no valid OSPF route to the ASBR's router ID), the Type 5 LSA is considered unreachable and is not installed, even though it exists in the OSPF database.
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 ASBR is not reachable via an OSPF internal route.
Why this is correct
OSPF computes external routes from type 5 LSAs only if the originating ASBR is reachable through an intra-area or inter-area route. The SPF algorithm builds a shortest-path tree to the ASBR first; if the ASBR's router LSA is missing or the resulting next hop has no valid OSPF route, the external prefix is left in the LSDB but never installed in the RIB. This is a fundamental adjacency-dependent rule for external route installation.
- ✗
The type 5 LSA has a metric of 16777215.
Why it's wrong here
The type 5 LSA metric field is 24 bits, so 16777215 is the maximum representable metric, not a special unreachable marker. OSPF does not use metric alone to suppress installation; even the highest metric produces a valid route. An external route fails to install solely because of ASBR unreachability or an SPF constraint, never because the absolute metric is too large.
- ✗
The OSPF process ID on the ASBR is different from the other routers.
Why it's wrong here
OSPF process IDs are purely local configuration values used to identify an OSPF instance on a router. They are not carried in OSPF protocol packets, router LSAs, or type 5 LSAs, and two neighbors can form a full adjacency even with different process IDs. Consequently, a mismatched process ID on the ASBR has no effect on LSA processing, LSDB synchronization, or route installation.
- ✗
The type 5 LSA is being filtered by an outbound route filter.
Why it's wrong here
An outbound route filter controls which LSAs are advertised to OSPF neighbors, not what is inserted into the local LSDB. If the type 5 LSA is already present in the LSDB, it has not been filtered from the database; filtering would prevent the LSA from ever being received or stored. The issue is therefore the ASBR's reachability within the OSPF domain, not a filter that would have removed the LSA before it entered the database.
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.
Go deeper
Related to this question
About these practice questions
This 350-401 question is part of Courseiva's 1,923-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco 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 350-401 exam.