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300-410 Practice Question: An OSPF network is experiencing routing loops…
An OSPF network is experiencing routing loops between two routers. R1 has the following configuration: interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.0 ip ospf network point-to-point. R2 has: interface GigabitEthernet0/0 ip address 10.1.1.2 255.255.255.0 ip ospf network broadcast. Both routers are in area 0. R1 shows: 'show ip ospf neighbor' lists R2 as FULL/DR, but R2 shows R1 as FULL/DROTHER. Traffic between two subnets behind each router is looping. 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 network type mismatch causes both routers to become DR/BDR, leading to incorrect LSA generation and routing loops.
OSPF network type mismatch on a multi-access link can cause adjacency issues. R1 is configured as point-to-point, which does not participate in DR/BDR election and expects a point-to-point link. R2 is configured as broadcast, which expects a DR/BDR election. This mismatch can cause both routers to form an adjacency (since OSPF still works), but they have different views of the network. R1 thinks the link is point-to-point and does not expect a DR, so it may advertise the link as a stub network or with different metrics. R2 thinks it is the DR (since R1 is not participating in election), and it advertises the link as a transit network. This inconsistency can cause routing loops because R1 may advertise a route via R2, and R2 may advertise the same route via R1, creating a loop. The correct fix is to match the network type on both ends.
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 network type mismatch causes both routers to become DR/BDR, leading to incorrect LSA generation and routing loops.
Why this is correct
R1's point-to-point configuration means it does not participate in DR election, so R2 becomes DR. R1 treats the link as a point-to-point link and may not advertise the correct type 2 LSA, while R2 advertises a type 2 LSA. This mismatch can cause R1 to learn routes via R2 and R2 to learn routes via R1, creating a loop.
- ✗
The IP addresses are on the same subnet, but the OSPF cost is set differently, causing unequal cost load balancing.
Why it's wrong here
Cost mismatch would not cause a loop; it would affect path selection.
- ✗
The OSPF hello and dead intervals are mismatched due to the network type, causing the adjacency to flap.
Why it's wrong here
The adjacency is FULL, so hello/dead intervals are not mismatched.
- ✗
The OSPF area is misconfigured; one router is in a different area.
Why it's wrong here
Both are in area 0, as stated.
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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