OSPFCCNA 200-301

OSPF Hello/Dead Timer Mismatch Preventing Adjacency

Presenting Symptom

OSPF neighbors are stuck in INIT state or never become FULL, and the network engineer observes that routers are not exchanging routing information.

Network Context

A small branch office with two Cisco routers (R1 and R2) connected via a point-to-point Ethernet link. Both routers run IOS 15.x and are configured for OSPF in area 0. The network uses a single subnet (192.168.1.0/24) between the routers. The problem prevents OSPF adjacency from forming, causing routing black holes.

Diagnostic Steps

Check OSPF neighbor status

show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.1.2      1    INIT/ -        00:00:32     192.168.1.2     GigabitEthernet0/0

If the neighbor is stuck in INIT state, it means the router has received a Hello packet but the parameters (like Hello/Dead timers) do not match. A normal state should be FULL or 2WAY.

Verify OSPF interface parameters on both routers

show ip ospf interface GigabitEthernet0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.1.1/24, 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, Interface address 192.168.1.1
  Backup Designated router (ID) 192.168.1.2, Interface address 192.168.1.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Note the Hello and Dead timer values. Compare them with the neighbor's values. A mismatch (e.g., Hello 10 vs Hello 30) will prevent adjacency.

Compare Hello/Dead timers on the neighbor router

show ip ospf interface GigabitEthernet0/0 (on R2)

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.1.2/24, Area 0
  Process ID 1, Router ID 192.168.1.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 192.168.1.1, Interface address 192.168.1.1
  Backup Designated router (ID) 192.168.1.2, Interface address 192.168.1.2
  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5

If the Hello or Dead timers differ between the two routers, this is the root cause. OSPF requires these timers to match for adjacency to form.

Check OSPF debug messages (optional, for deeper analysis)

debug ip ospf hello

OSPF: Mismatched hello parameters from 192.168.1.2
  Dead R 40 C 120, Hello R 10 C 30  Mask R 255.255.255.0 C 255.255.255.0

The debug output explicitly shows mismatched Hello and Dead timers. 'R' indicates received values from the neighbor, 'C' indicates configured values on the local router. This confirms the mismatch.

Root Cause

The OSPF Hello and Dead timers are mismatched between the two routers. R1 has Hello 10 seconds and Dead 40 seconds (default for broadcast), while R2 has Hello 30 seconds and Dead 120 seconds (likely manually configured or default for non-broadcast). OSPF requires these timers to be identical on both ends of a link for adjacency to form.

Resolution

On R2, change the Hello and Dead timers to match R1's default values: R2(config)# interface GigabitEthernet0/0 R2(config-if)# ip ospf hello-interval 10 R2(config-if)# ip ospf dead-interval 40 Alternatively, on R1, change to match R2's values, but using the default broadcast timers is recommended. The dead-interval should be four times the hello-interval by default.

Verification

After applying the fix, verify adjacency forms: R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/DR 00:00:35 192.168.1.2 GigabitEthernet0/0 Also verify timers match: R1# show ip ospf interface GigabitEthernet0/0 | include Timer Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 R2# show ip ospf interface GigabitEthernet0/0 | include Timer Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Prevention

1. Use consistent OSPF configuration templates across all routers to avoid manual timer changes. 2. Always verify OSPF interface parameters after configuration changes using 'show ip ospf interface'. 3. Avoid changing Hello/Dead timers unless required by the network design (e.g., for slow links).

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears as a troubleshooting question where you must identify why OSPF neighbors are not forming. The exam may present a 'show ip ospf neighbor' output showing INIT state and ask for the root cause. Key fact: OSPF Hello and Dead timers must match between neighbors. The exam also tests the default timer values (Hello 10s, Dead 40s for broadcast; Hello 30s, Dead 120s for non-broadcast).

Exam Tips

Remember that OSPF neighbors stuck in INIT state often indicate a parameter mismatch, such as Hello/Dead timers, area ID, or authentication.

The default Hello timer for broadcast and point-to-point networks is 10 seconds; Dead timer is 40 seconds. For NBMA, default Hello is 30 seconds, Dead is 120 seconds.

Use 'show ip ospf interface' to quickly verify timer values and other OSPF parameters on an interface.

Commands Used in This Scenario

show ip ospf interface

Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.

show ip ospf neighbor

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

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