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Passive-interface default command

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

The passive-interface command configures a specified OSPF-enabled interface to avoid sending OSPF messages for neighbor formation.

The passive-interface default command makes every OSPF-enabled interface passive by default. Passive interfaces remain OSPF-enabled and advertise their networks, but they do not send OSPF hello messages to form neighbors. section 18.2.3 section 18.5

Use the command sequence when only a few interfaces should form OSPF neighbor relationships:

  • Enter OSPF router configuration mode with router ospf process-id. section 18.2.3
  • Apply passive-interface default to make all interfaces passive. section 18.2.3
  • Use no passive-interface interface-name on each interface that must establish OSPF neighbors. section 18.2.3 section 18.5

R2 with selective OSPF neighbors

R2(config)# router ospf 1 R2(config-router)# passive-interface default R2(config-router)# no passive-interface g0/0 R2(config-router)# no passive-interface g0/1 G0/0 and G0/1 can form OSPF neighbor relationships, while Loopback0 remains passive.

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Explains default behavior and the reversal command, then demonstrates selective interface configuration without deeper OSPF internals.
Examples
Uses R2 commands to leave Loopback0 passive while making G0/0 and G0/1 nonpassive.
What it assumes
Assumes basic OSPF interface concepts while defining passive behavior and neighbor formation through configuration examples.
What you can do after
Operate/configure OSPF by making interfaces passive by default and selectively enabling neighbor formation.
How it connects
Connects passive interfaces with OSPF, hello messages, network prefixes, and neighbor relationships.
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