CCNA IP Routing Practice Question
Exhibit
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 EXSTART/EXCHANGE 00:00:35 10.1.1.2 GigabitEthernet0/0
R1# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0, Attached via Network Statement
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
Backup Designated router (ID) 2.2.2.2, Interface address 10.1.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Can be used as a virtual-link endpoint
Index 1/1/1, runqueue 0x0
Neighbor Count is 1, Adjacent neighbor count is 0
Adjacent with neighbor 2.2.2.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
R1# show running-config | section router ospf
router ospf 1
router-id 1.1.1.1
network 10.1.1.0 0.0.0.255 area 0
passive-interface default
no passive-interface GigabitEthernet0/0An engineer is troubleshooting an OSPF adjacency issue between two Cisco routers, R1 and R2, connected via GigabitEthernet0/0 on both sides. Hosts on R1's LAN cannot ping hosts on R2's LAN. The engineer checks the OSPF neighbor state on R1 and sees the adjacency is stuck in EXSTART/EXCHANGE. The router IDs are 1.1.1.1 on R1 and 2.2.2.2 on R2, and both routers have a network statement for their directly connected subnet. What is the most likely cause of this problem?
⚠ Common exam trap
Cisco often tests the MTU mismatch scenario by presenting symptoms of a stuck adjacency in EXSTART/EXCHANGE, which candidates may confuse with other issues like mismatched areas or passive interfaces, but the key clue is the specific state where DBD exchange fails.
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
✓
There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.
The EXSTART/EXCHANGE state indicates that OSPF neighbors have progressed past the 2-Way state and are attempting to exchange Database Description (DBD) packets. An MTU mismatch on the directly connected GigabitEthernet0/0 interfaces prevents successful DBD exchange because the interface with the smaller MTU drops the larger DBD packet, causing the adjacency to remain stuck in EXSTART/EXCHANGE. Options A and C are incorrect because passive-interface default and area mismatches prevent the adjacency from reaching EXSTART/EXCHANGE (they block Hello exchange), and option D is incorrect because conflicting router IDs would prevent adjacency formation entirely, not result in a stuck EXSTART/EXCHANGE state.
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 passive-interface default command is blocking OSPF Hellos on GigabitEthernet0/0.
Why it's wrong here
If `passive-interface default` had been configured under `router ospf`, all interfaces would suppress Hello packets. However, the running configuration includes the `no passive-interface GigabitEthernet0/0` statement, specifically re-enabling Hellos on that link. Additionally, the neighbor relationship progressing to EXSTART proves Hellos are being exchanged, so passive interference is not the problem.
- ✓
There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.
Why this is correct
The adjacency is stuck in EXSTART/EXCHANGE because R1 and R2 have mismatched MTU values on their GigabitEthernet0/0 interfaces. During the Database Description exchange, OSPF includes the interface MTU in the DBD packet; when one router sends a DBD larger than the peer's MTU, the peer silently drops it, so the routers never finish negotiating master/slave. The fix is configuring the same MTU, for example `ip mtu 1500`, on both sides to allow DBD packets to pass.
- ✗
The OSPF area configured on the interface does not match between R1 and R2.
Why it's wrong here
An OSPF area mismatch would cause the routers to discard each other's Hello packets because the Area ID is carried inside every Hello message. R1 and R2 both advertise network 10.1.1.0/24 as belonging to area 0, so their interfaces share the same area membership. Because the neighbor state has advanced beyond 2-Way, Hello processing is already succeeding, making this option impossible.
- ✗
The router IDs 1.1.1.1 and 2.2.2.2 are conflicting with each other.
Why it's wrong here
OSPF router IDs must be unique within the routing domain, but 1.1.1.1 and 2.2.2.2 are clearly different addresses. A conflict would only occur if both routers used the same router ID, causing the OSPF process to be unable to distinguish them. Since these are distinct, this option is not the cause of the adjacency failure.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.Correct answer▾
Why this is correct
The adjacency is stuck in EXSTART/EXCHANGE because R1 and R2 have mismatched MTU values on their GigabitEthernet0/0 interfaces. During the Database Description exchange, OSPF includes the interface MTU in the DBD packet; when one router sends a DBD larger than the peer's MTU, the peer silently drops it, so the routers never finish negotiating master/slave. The fix is configuring the same MTU, for example `ip mtu 1500`, on both sides to allow DBD packets to pass.
✗The passive-interface default command is blocking OSPF Hellos on GigabitEthernet0/0.Wrong answer — click to see why▾
Why this is wrong here
The passive-interface default command would suppress OSPF Hellos on all interfaces, preventing the adjacency from forming at all, not just getting stuck in EXSTART/EXCHANGE. The adjacency would remain in DOWN or INIT state, not EXSTART/EXCHANGE.
Why candidates choose this
Students may confuse the effect of passive-interface with causing adjacency issues, but the specific state EXSTART/EXCHANGE indicates that Hellos have been exchanged and the routers are trying to synchronize databases, which requires DBD packets to be sent.
✗The OSPF area configured on the interface does not match between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
An area mismatch would prevent the adjacency from forming at all, typically resulting in the neighbor state remaining in INIT or 2-WAY, not EXSTART/EXCHANGE. The question states both routers have a network statement for the directly connected subnet, implying the same area is configured.
Why candidates choose this
Area mismatch is a common OSPF issue, but it manifests earlier in the adjacency process. Students might assume any adjacency problem is due to area mismatch without considering the specific state.
✗The router IDs 1.1.1.1 and 2.2.2.2 are conflicting with each other.Wrong answer — click to see why▾
Why this is wrong here
Router IDs must be unique for OSPF to form an adjacency. Here, the router IDs are different (1.1.1.1 and 2.2.2.2), so there is no conflict. A conflict would cause the adjacency to fail, but not specifically stuck in EXSTART/EXCHANGE.
Why candidates choose this
Students may think that router IDs need to be in the same subnet or similar, but OSPF only requires uniqueness. The specific state EXSTART/EXCHANGE points to a DBD packet issue, not a router ID conflict.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Go deeper
Related to this question
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
Key term
OSPF adjacency
An OSPF adjacency is a logical neighbor relationship formed between two OSPF routers that have completed a series of hello and database exchange processes, enabling them to share routing information and maintain a consistent view of the network topology.
Key term
LAN
Local Area Network — a network confined to a single physical location such as an office, building, or campus.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.