Question 1,761 of 2,152

Route Redistribution →mediumMultiple ChoiceObjective-mapped

How to Verify OSPF External Routes from Redistribution Using show ip ospf database external

This 300-410 practice question tests your understanding of route redistribution. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A network engineer runs the following command to troubleshoot a Route Redistribution issue:

R1# show ip ospf database external

And sees the following output:

OSPF Router with ID (1.1.1.1) (Process ID 1)

Type-5 AS External Link States

LS age: 360 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 192.168.10.0 (External Network Number ) Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0x1234 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 100

What does this output indicate?

A
The route 192.168.10.0/24 is an OSPF internal route.
Why wrong: The route is an external route, not internal, as indicated by the Type 5 LSA. Internal OSPF routes are Type 1 or Type 2 LSAs.
B
The route is redistributed into OSPF with a metric of 20 and a route tag of 100.
Why wrong: The route is redistributed but it is a default route, not a specific network route. The metric and tag are correct, but the route type is default.
C
The forward address is 0.0.0.0, meaning the route is not reachable.
Why wrong: A forward address of 0.0.0.0 does not mean the route is unreachable; it means the advertising router is the next hop. The route is reachable via the advertising router.
D
The route is a default route redistributed into OSPF.
Correct. The Type 5 LSA with forward address 0.0.0.0 and metric type 2 is characteristic of a default route redistributed into OSPF, as default routes often have a forward address of 0.0.0.0 and a route tag for identification.

Full breakdown with real-world context →

Quick Answer

The answer is that the route is redistributed into OSPF with a metric of 20 and a route tag of 100. This is correct because the Type 5 LSA output shows a metric type of 2 with a value of 20, which is the default cost for routes redistributed from external protocols like EIGRP or BGP, and the External Route Tag of 100 confirms administrative tagging during redistribution. On the Cisco CCNP ENARSI 300-410 exam, this command tests your ability to verify OSPF external routes from redistribution by interpreting the LSA fields, particularly distinguishing between Type 5 and Type 7 LSAs or spotting a non-zero Forward Address, which indicates a different redistribution scenario. A common trap is assuming a metric of 20 always means a directly connected route, but here the Type 2 metric and tag clearly point to redistribution. Memory tip: "Tag 100, metric 20—redistributed, not friendly."

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 route is a default route redistributed into OSPF.

The output shows a Type 5 LSA with a forward address of 0.0.0.0 and metric type 2. In OSPF, a forward address of 0.0.0.0 in a Type 5 LSA indicates that the advertising router is the next hop, and this is commonly observed for default routes redistributed into OSPF. Additionally, the external route tag of 100 is often used to mark default routes for filtering. Therefore, the route is a default route redistributed into OSPF.

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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 route 192.168.10.0/24 is an OSPF internal route.
Why it's wrong here
The route is an external route, not internal, as indicated by the Type 5 LSA. Internal OSPF routes are Type 1 or Type 2 LSAs.
✗
The route is redistributed into OSPF with a metric of 20 and a route tag of 100.
Why it's wrong here
The route is redistributed but it is a default route, not a specific network route. The metric and tag are correct, but the route type is default.
✗
The forward address is 0.0.0.0, meaning the route is not reachable.
Why it's wrong here
A forward address of 0.0.0.0 does not mean the route is unreachable; it means the advertising router is the next hop. The route is reachable via the advertising router.
✓
The route is a default route redistributed into OSPF.
Why this is correct
Correct. The Type 5 LSA with forward address 0.0.0.0 and metric type 2 is characteristic of a default route redistributed into OSPF, as default routes often have a forward address of 0.0.0.0 and a route tag for identification.
Related concept
OSPF neighbours must agree on key parameters.

Common exam traps

Common exam trap: OSPF can fail even when IP connectivity looks correct

OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.

Detailed technical explanation

How to think about this question

OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

Key takeaway

OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

Real-world example

How this comes up in practice

A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

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.

What to study next

Got this wrong? Here's your next step.

Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Route Redistribution — This question tests Route Redistribution — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The route is a default route redistributed into OSPF. — The output shows a Type 5 LSA with a forward address of 0.0.0.0 and metric type 2. In OSPF, a forward address of 0.0.0.0 in a Type 5 LSA indicates that the advertising router is the next hop, and this is commonly observed for default routes redistributed into OSPF. Additionally, the external route tag of 100 is often used to mark default routes for filtering. Therefore, the route is a default route redistributed into OSPF.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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