Question 1,114 of 1,958
mediumMultiple ChoiceObjective-mapped
OSPF Type 5 LSA Forwarding Address Reachability and Route Installation
An enterprise network uses OSPF as its IGP. The network engineer notices that a particular route learned via OSPF is not being installed in the routing table, even though the neighbor adjacency is up and the route appears in the OSPF database. The route is an external route redistributed from EIGRP. What is the most likely cause?
Quick Answer
The answer is that the forwarding address in the type 5 LSA is not reachable via an OSPF internal route. OSPF enforces a strict reachability check before installing external routes: the forwarding address (FA) field in a Type 5 LSA must be reachable through an intra-area or inter-area OSPF route. If the FA points to a prefix that is only reachable via another external route or is simply absent from the OSPF domain, the router will ignore the LSA entirely, keeping the route out of the routing table despite the LSA being present in the database and the neighbor adjacency being up. On the ENCOR 350-401 exam, this concept tests your understanding of OSPF external route installation rules, often appearing as a trick where candidates assume a full database means a full routing table. A common trap is forgetting that the FA is often set to 0.0.0.0 (meaning the originating ASBR is the next hop) but when non-zero, it must be an internal OSPF route. Memory tip: FA must be FA-miliar to OSPF internally, or the route stays out.
⚠ Common exam trap
Cisco often tests the forwarding address reachability requirement for Type 5 LSAs, and the trap here is that candidates assume any route in the OSPF database will automatically be installed, ignoring the recursive lookup condition for external routes with a non-zero forwarding address.
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 forwarding address in the type 5 LSA is not reachable via an OSPF internal route.
OSPF requires the forwarding address (FA) in a Type 5 LSA to be reachable via an OSPF internal route (intra-area or inter-area) for the external route to be installed in the routing table. If the FA is not reachable, the router will ignore the LSA and not install the route, even though the LSA exists in the OSPF database and the neighbor adjacency is up.
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 OSPF process ID is different on the routers.
Why it's wrong here
OSPF process IDs are locally significant and do not need to match between neighbours for adjacency formation or route exchange; the router ID, area configuration, and authentication parameters govern adjacency, not the process ID. This option is tempting because many routing protocols, such as EIGRP, require matching autonomous system numbers for neighbourship, so a candidate might incorrectly assume OSPF behaves identically.
- ✗
The external route has a higher administrative distance than the internal route.
Why it's wrong here
Incorrect because administrative distance is used when comparing routes from different sources; the OSPF external route is not being installed at all, not being compared.
- ✓
The forwarding address in the type 5 LSA is not reachable via an OSPF internal route.
Why this is correct
Correct because OSPF requires the forwarding address to be reachable via an intra-area or inter-area route; otherwise, the external route is not installed.
- ✗
The OSPF metric for the external route is too high.
Why it's wrong here
Incorrect because OSPF does not have a maximum metric that prevents route installation; even high metrics are installed.
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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Same concept, more angles
1 more way this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
hard- ✓ A.The ASBR is not reachable via an OSPF internal route.
- B.The type 5 LSA has a metric of 16777215.
- C.The OSPF process ID on the ASBR is different from the other routers.
- D.The type 5 LSA is being filtered by an outbound route filter.
Why A: 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.
Last reviewed: Jun 24, 2026
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