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300-410 Practice Question: An enterprise uses BGP to exchange routes with a…
An enterprise uses BGP to exchange routes with a service provider. After redistributing OSPF into BGP, some prefixes are missing. Router R1 config:
router bgp 65001
redistribute ospf 1 match internal external 1 external 2 !
router ospf 1
redistribute bgp 65001 subnets
R1# show ip bgp 10.10.10.0
% Network not in table
R1# show ip ospf database external 10.10.10.0
Type-5 AS External Link States LS age: 360 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.10.10.0 (External Network Number) Advertising Router: 192.168.1.2 LS Seq Number: 80000001 Checksum: 0x1234 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0
What is the root 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 OSPF external route is not installed in the routing table because of a mismatched metric type or LSA filtering.
The OSPF external route 10.10.10.0/24 has a Forward Address of 0.0.0.0, which means it is reachable via the advertising router. However, BGP redistribution of OSPF requires the route to be in the OSPF routing table, not just the database. The route may be missing due to an OSPF LSA filtering or a mismatched area. The correct fix is to ensure the OSPF route is fully installed and not filtered.
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 external route is not installed in the routing table because of a mismatched metric type or LSA filtering.
Why this is correct
The route appears in the database but not in the routing table, so BGP cannot redistribute it.
- ✗
The BGP redistribute command is missing the route-map to set the metric.
Why it's wrong here
Missing route-map does not prevent redistribution; the route must be in the routing table.
- ✗
The OSPF process is not configured with the default-information originate command.
Why it's wrong here
Default route is not needed; the external route is specific.
- ✗
The BGP table is full and cannot accept more routes.
Why it's wrong here
No evidence of table overflow; the route is simply not in the table.
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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