hardMultiple ChoiceObjective-mapped
220-1201 Practice Question: A network administrator configures a router to…
A network administrator configures a router to use OSPF for dynamic routing between two branch offices. After configuration, routes from one office are not appearing in the other's routing table. Both offices can ping each other's directly connected interfaces. Which protocol issue is most likely preventing route exchange?
⚠ Common exam trap
A common mistake is to think that OSPF process IDs must match between routers. The trap here is that candidates overlook the multicast dependency. While ping success eliminates basic Layer 3 connectivity issues like mismatched subnet masks, it does not eliminate OSPF parameter mismatches such as area IDs, which are checked after Hello packets are received.
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
✓
Multicast is not being forwarded between the routers.
OSPF uses multicast address 224.0.0.5 (AllSPFRouters) to exchange hello packets and LSAs. If multicast forwarding is disabled or blocked between the routers (e.g., by an ACL, switchport configuration, or lack of multicast routing), OSPF neighbors will not form, and routes will not be exchanged. Since both offices can ping each other's directly connected interfaces, Layer 3 unicast connectivity exists, but OSPF's multicast-based neighbor discovery is failing.
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 routers are using different OSPF area IDs.
Why it's wrong here
OSPF routers must belong to the same area on a shared network segment to establish a neighbor adjacency. If two directly connected routers are configured with different OSPF area IDs on their interfaces connected to the same segment, they will not form a neighbor relationship. This mismatch prevents the exchange of OSPF Hellos and subsequent database synchronization, effectively stopping OSPF from operating between them on that link. While OSPF supports multiple areas, this specific configuration on a shared segment would directly impede neighbor discovery.
- ✗
The routers are using different OSPF process IDs.
Why it's wrong here
The OSPF process ID is a locally significant identifier used by a router to distinguish between multiple OSPF instances running on the same device. It serves administrative purposes only and does not need to match between neighboring routers for them to form an adjacency. Neighbor discovery and adjacency formation rely on other parameters, such as area ID, authentication, and network type, not the process ID.
- ✓
Multicast is not being forwarded between the routers.
Why this is correct
OSPF routers primarily use multicast addresses 224.0.0.5 (All OSPF Routers) and 224.0.0.6 (All OSPF DR/BDRs) to send Hello packets for neighbor discovery and to exchange link-state advertisements (LSAs) on broadcast multi-access networks. If multicast traffic is not being forwarded or is being blocked between the routers, they will be unable to discover each other as neighbors or exchange routing information. This directly prevents OSPF from establishing adjacencies and building its routing table.
- ✗
The routers have mismatched subnet masks on their interfaces.
Why it's wrong here
Mismatched subnet masks on directly connected interfaces would place the routers in different logical subnets, even if they share the same physical link. This configuration would prevent direct IP communication between them, including basic ICMP pings. Since the question implies that basic connectivity (like pings) is functional, mismatched subnet masks are not the underlying cause of the OSPF neighbor formation failure.
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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