CCNA IP Routing Practice Question
Exhibit
R1# show ipv6 ospf neighbor
Neighbor ID Pri State Dead Time Interface ID Interface
192.168.1.2 1 EXSTART/DR 00:00:32 2 GigabitEthernet0/0
R1# show ipv6 ospf interface gigabitethernet0/0
GigabitEthernet0/0 is up, line protocol is up
Link Local Address FE80::1, Interface ID 2
Area 0, Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.1.1, local address FE80::1
Backup Designated router (ID) 192.168.1.2, local address FE80::2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:07
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 0
Neighbor Count is 1, Adjacent neighbor count is 0
Adjacent with neighbor 192.168.1.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
R1# show ipv6 route ospf
<no output>A network engineer is troubleshooting an OSPFv3 adjacency issue between two directly connected routers. Both routers are configured for OSPFv3 in area 0 on their GigabitEthernet0/0 interfaces. The engineer checks the OSPFv3 neighbor status on R1 and sees that the neighbor state is stuck in EXSTART. The engineer verifies that both interfaces are up and have IPv6 link-local addresses. What is the most likely cause of this problem?
⚠ Common exam trap
Cisco often tests the MTU mismatch as a specific cause for OSPF adjacency being stuck in EXSTART, and candidates may mistakenly focus on router IDs or addressing instead of the packet size negotiation.
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
✓
Ensure that the MTU is the same on both sides of the link.
The EXSTART state in OSPF indicates that the routers have formed a bidirectional communication and are negotiating the master/slave relationship and the initial Database Description (DBD) packet exchange. A common cause for getting stuck in EXSTART is an MTU mismatch between the two interfaces, which prevents the DBD packets from being accepted by the neighbor, causing the process to stall.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure a global unicast IPv6 address on the interface.
Why it's wrong here
OSPFv3's Hello protocol and all neighbor state exchanges operate using IPv6 link-local addresses, which are automatically derived on each interface; a global unicast address is not a prerequisite for forming an adjacency. Without a global address, OSPFv3 can still discover neighbors and exchange routing information. Therefore, adding a global unicast address will not address an adjacency failure caused by an MTU mismatch, since the link-local path remains unaffected.
- ✓
Ensure that the MTU is the same on both sides of the link.
Why this is correct
An MTU mismatch prevents OSPFv3 from completing the DBD packet exchange, leaving the neighbor state stuck in EXSTART. During this phase, routers send Database Description packets that must fit within the interface MTU; if one side's MTU is lower, the packets are dropped, and the routers keep retransmitting without progressing to EXCHANGE. Aligning the MTU on both ends allows the DBD exchange to complete, moving the adjacency to FULL.
- ✗
Change the router ID to be the same on both routers.
Why it's wrong here
OSPFv3 relies on the Router ID as the unique identifier for each router in the routing domain. Setting the same Router ID on both routers makes each see the other as a duplicate neighbor, causing the adjacency to be rejected or to flap continuously. Since correct operation demands distinct Router IDs, making them identical would only worsen the situation, not solve the MTU-related problem.
- ✗
Change the network type to point-to-point.
Why it's wrong here
OSPFv3 defaults to broadcast network type over Ethernet, where neighbors automatically elect a DR/BDR and still perform full DBD exchanges. Switching to point-to-point merely suppresses DR/BDR election; it does not alter the size or handling of DBD packets. Because the underlying MTU mismatch remains, the adjacency will still get stuck in EXSTART even in point-to-point mode.
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.
✓Ensure that the MTU is the same on both sides of the link.Correct answer▾
Why this is correct
An MTU mismatch prevents OSPFv3 from completing the DBD packet exchange, leaving the neighbor state stuck in EXSTART. During this phase, routers send Database Description packets that must fit within the interface MTU; if one side's MTU is lower, the packets are dropped, and the routers keep retransmitting without progressing to EXCHANGE. Aligning the MTU on both ends allows the DBD exchange to complete, moving the adjacency to FULL.
✗Configure a global unicast IPv6 address on the interface.Wrong answer — click to see why▾
Why this is wrong here
The problem is not due to missing global addresses; the adjacency is stuck at EXSTART, not at DOWN or INIT.
✗Change the router ID to be the same on both routers.Wrong answer — click to see why▾
Why this is wrong here
The show output shows different router IDs (192.168.1.1 and 192.168.1.2), so this is not the issue.
✗Change the network type to point-to-point.Wrong answer — click to see why▾
Why this is wrong here
The adjacency is stuck in EXSTART, which is not typically resolved by changing network type; the issue is related to packet exchange.
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
Area 0
Area 0 is the backbone area in OSPF routing that connects all other OSPF areas to ensure a loop-free and efficient network routing topology.
Key term
IPv6 link-local
An IPv6 link-local address is a self-assigned, non-routable address used for communication between devices on the same network segment without needing a central server.
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JA
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.