IPv6CCNA 200-301

OSPFv3 Neighbor Not Forming in IPv6 Network

Presenting Symptom

OSPFv3 neighbors are not forming between two directly connected routers in an IPv6 network.

Network Context

Two Cisco routers (R1 and R2) are directly connected via GigabitEthernet0/0 in a small branch office. Both routers run IOS 15.9 and have IPv6 enabled on the interface. OSPFv3 is configured with process ID 1 and area 0. The routers are not forming an adjacency, and the 'show ipv6 ospf neighbor' command shows no neighbors.

Diagnostic Steps

Check OSPFv3 neighbor status

show ipv6 ospf neighbor

Neighbor ID     Pri   State           Dead Time   Interface ID    Interface
   (no output or empty)

If no neighbors are listed, the adjacency has not formed. This indicates a problem with OSPFv3 configuration or interface issues.

Verify OSPFv3 interface configuration

show ipv6 ospf interface gigabitethernet 0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address FE80::1, Area 0.0.0.0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  No Hellos (Passive interface)

Check if the interface is OSPF-enabled and if it is passive. If 'No Hellos (Passive interface)' appears, OSPF hellos are not being sent, preventing neighbor discovery.

Check OSPFv3 passive interface configuration

show running-config | section router ospf

router ospf 1
  passive-interface default
  no passive-interface GigabitEthernet0/0

If 'passive-interface default' is configured without 'no passive-interface' for the connecting interface, the interface will not send OSPF hellos. This is a common misconfiguration.

Verify IPv6 link-local addresses and interface status

show ipv6 interface brief

GigabitEthernet0/0   [up/up]
    FE80::1

Ensure the interface is up/up and has a link-local address. If the interface is down or has no link-local address, OSPF cannot form adjacency.

Root Cause

The OSPFv3 interface is configured as passive under the OSPF process due to 'passive-interface default' command, which prevents the interface from sending OSPF hello packets. Without hellos, the routers cannot discover each other and form an adjacency.

Resolution

Remove the passive-interface configuration on the connecting interface: R1(config)# router ospf 1 R1(config-router)# no passive-interface GigabitEthernet0/0 This command allows the interface to send OSPF hello packets, enabling neighbor discovery and adjacency formation.

Verification

After applying the fix, verify neighbor formation: R1# show ipv6 ospf neighbor Expected output: Neighbor ID Pri State Dead Time Interface ID Interface 2.2.2.2 1 FULL/DR 00:00:39 2 GigabitEthernet0/0 Also verify OSPF interface is no longer passive: R1# show ipv6 ospf interface gigabitethernet 0/0 | include passive (no output indicates interface is not passive)

Prevention

1. Avoid using 'passive-interface default' unless you explicitly enable OSPF on required interfaces with 'no passive-interface'. 2. Use 'passive-interface' only on interfaces that should not form OSPF adjacencies (e.g., toward end hosts). 3. Always verify OSPF neighbor formation after configuration changes using 'show ipv6 ospf neighbor'.

CCNA Exam Relevance

On the CCNA 200-301 exam, OSPFv3 troubleshooting appears in multiple-choice and simulation questions. The exam tests understanding of OSPFv3 configuration, neighbor states, and common issues like passive interfaces. A key fact is that OSPFv3 uses link-local addresses for neighbor discovery and requires hellos to be sent on the interface.

Exam Tips

Remember that 'passive-interface default' makes all interfaces passive; you must use 'no passive-interface' to enable OSPF on specific interfaces.

OSPFv3 neighbor states progress from DOWN to INIT to TWO-WAY to EXSTART to EXCHANGE to LOADING to FULL. If stuck in INIT, check hello parameters.

The 'show ipv6 ospf interface' command is critical for verifying OSPFv3 interface settings, including network type, cost, and passive status.

Commands Used in This Scenario

show ipv6 interface brief

Displays a summary of IPv6 interface status and addresses, useful for quickly verifying IPv6 configuration and interface operational state.

show running-config

Displays the current active configuration in DRAM, showing all non-default settings.

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