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OSPFCCNA 200-301

OSPF Neighbor Stuck in EXSTART/EXCHANGE State

Presenting Symptom

OSPF neighbor adjacency is stuck in EXSTART/EXCHANGE state and does not transition to FULL.

Network Context

Two Cisco routers (R1 and R2) are directly connected over a FastEthernet link in a small branch office. Both run IOS 15.x. The OSPF process is configured with network type broadcast on the interface. The problem occurs after a recent configuration change on R2's MTU.

Diagnostic Steps

1

Check OSPF neighbor state

show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.2         1   EXSTART/DR      00:00:32    192.168.1.2     GigabitEthernet0/0

If neighbor is stuck in EXSTART or EXCHANGE, it indicates a problem during database exchange. Normal state should be FULL.

2

Verify interface MTU on both sides

show interfaces GigabitEthernet0/0 | include MTU
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

Compare MTU values on both routers. If they differ, OSPF will fail to exchange database packets. The MTU must match on both ends.

3

Check OSPF interface parameters

show ip ospf interface GigabitEthernet0/0
Internet Address 192.168.1.1/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 10.0.0.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 10.0.0.1, Interface address 192.168.1.1
  Backup Designated router (ID) 10.0.0.2, Interface address 192.168.1.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Ensure network type is broadcast and timers match. Mismatched timers can cause issues but typically not EXSTART/EXCHANGE. Focus on MTU.

4

Debug OSPF adjacency events

debug ip ospf adj
OSPF: Rcv pkt from 192.168.1.2, GigabitEthernet0/0 : Mismatched OSPF packet type: Database Description with MTU mismatch

If debug shows 'Mismatched ... MTU mismatch', the root cause is confirmed. The router receiving a Database Description packet with an MTU larger than its own will drop the packet.

Root Cause

The MTU on the interface of one router (R2) is set to a different value (e.g., 1400) than the other router (R1, MTU 1500). OSPF uses the interface MTU to determine the maximum size of Database Description packets. When a router receives a DBD packet larger than its own MTU, it drops it, causing the adjacency to remain stuck in EXSTART/EXCHANGE.

Resolution

Set the MTU to the same value on both interfaces. On R1: interface GigabitEthernet0/0 ip mtu 1500 On R2: interface GigabitEthernet0/0 ip mtu 1500 Alternatively, if the link has a lower MTU, set both to that value. After changing MTU, clear the OSPF process to re-establish adjacency: clear ip ospf process

Verification

Run 'show ip ospf neighbor' on R1. Expected output: Neighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 FULL/DR 00:00:36 192.168.1.2 GigabitEthernet0/0 Also verify MTU matches with 'show interfaces | include MTU' on both routers.

Prevention

1. Standardize MTU across all interfaces in the OSPF domain, typically 1500 bytes for Ethernet. 2. Use 'ip mtu' command consistently when configuring interfaces. 3. Avoid changing MTU on OSPF-enabled interfaces without verifying the neighbor's MTU.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why OSPF neighbors are stuck in EXSTART/EXCHANGE. The exam tests knowledge of OSPF neighbor states and the impact of MTU mismatch. A candidate must know that MTU must match for OSPF adjacency to form fully.

Exam Tips

1.

Remember that EXSTART/EXCHANGE state indicates a problem during database description packet exchange, often due to MTU mismatch.

2.

The 'show ip ospf neighbor' command is key to identify the stuck state; 'debug ip ospf adj' can confirm MTU mismatch.

3.

On the exam, if you see an OSPF neighbor stuck in EXSTART, immediately check interface MTU on both routers.

Commands Used in This Scenario

clear ip ospf process

Resets the OSPF process on the router, forcing it to re-establish all neighbor adjacencies and re-learn routes.

debug ip ospf adj

Use this command to debug OSPF adjacency events in real-time, helping to troubleshoot why OSPF neighbors are not forming or are flapping.

show interfaces

Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.

show ip ospf neighbor

Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.

Test Your CCNA Knowledge

Practice with scenario-based questions to prepare for the CCNA 200-301 exam.

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