OSPF Passive Interfaces: Configuration and Best Practices (Updated 2025)

In OSPF (Open Shortest Path First), OSPF passive interface is a configuration that prevents the router from sending or receiving OSPF Hello packets on a specific interface while still allowing the connected network to be advertised in OSPF updates. This is crucial for CCNA and CCNP students to understand as it optimizes bandwidth, enhances security, and prevents unnecessary adjacencies on stub networks like LANs connected to hosts.

Passive interfaces are especially useful in scenarios where an interface connects to non-OSPF devices, such as end-user subnets, to avoid wasting resources on multicast Hellos (224.0.0.5/6).

OSPF messages are forwarded to all OSPF-enabled interfaces by default. However, some interfaces do not need these messages. When you configure the OSPF protocol on an interface using the network command, two things will occur:

• All interfaces configured with a network command will be advertised in OSPF.
• OSPF hello packets are sent on these interfaces periodically.

Router1, Router2, Router3, and Router4 have been configured with OSPF protocols. If Router1, connected to the network 192.168.10.0 /24, which has no OSPF neighbour. Router1 also wants to advertise this network to other routers.

Once the router is configured with a network command to advertise 192.168.10.0 /24 in OSPF, Router1 will also send OSPF hello packets towards 192.168.10.0/24, where no OSPF-enabled router is available. So, this is not a good idea because of:

• Inefficient Use of Bandwidth – The unwanted OSPF multicast messages consumed network bandwidth.
• Inefficient Use of Resources – The devices on the network also process the message and eventually discard the message.
• Increased Security Risk – Advertising updates on a broadcast network is also a security risk. If someone on the computer starts an application that replies with OSPF hello packets, then Router1 will try to become neighbours. An attacker could advertise fake routes using this technique that misdirect traffic.

Configuring OSPF Passive Interfaces

We can prevent this by configuring “passive-interface”. We can configure OSPF passive interfaces using the “passive-interface” command in router configuration mode.

This command tells OSPF not to send hello packets on the desired interfaces. But still, allow that network to be advertised to other routers. Below is the configuration of the passive interfaces.

Router1> enable
Router1# configure terminal
Router1(config)# interface GigabitEthernet0/0
Router1(config-if)# ip address 192.168.10.1 255.255.255.0
Router1(config-if)# no shutdown
Router1(config-if)# exit
Router1(config)# router ospf 20
Router1(config-router)# network 192.168.10.0 0.0.0.255 area 0
Router1(config-router)# passive-interface GigabitEthernet0/0
Router1(config-router)# end
Router1# copy running-config startup-config

OSPF Passive interfaces accept routing updates but do not send them, as shown below. Specifically, the passive-interface command stops routing messages from sending out the particular interface.

But, the network that the particular interface belongs to is still advertised in routing messages that sent out other interfaces. The OSPF passive interfaces cannot form a neighbour adjacency. Because link-state packets cannot be sent or acknowledged over the OSPF passive interfaces.

The show ip protocols command verify the passive-interface configuration, as shown below. The GigabitEthernet 0/0 interface is now listed under the Passive Interface(s) section but network 192.168.10.0 is still listed under Routing for Networks, which means that this network is still incorporated as a route entry in OSPF updates.

Router1# show ip protocols
*** Output omitted for brevity ***
Routing Protocol is "ospf 20"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 10.10.10.1
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    192.168.10.0 0.0.0.255 area 0
    10.1.1.0 0.0.0.3 area 0
    10.1.1.4 0.0.0.3 area 0
  Passive Interface(s):
    GigabitEthernet0/0
  Routing Information Sources:
    Gateway         Distance      Last Update
    10.1.1.2        110           00:00:22
    10.1.1.6        110           00:00:25
  Distance: (default is 110)

The passive-interface command is similar for both IPv4 (OSPFv2) and IPv6 (OSPFv3), but in OSPFv3, it’s under “ipv6 router ospf <process>”.

Best Practices for OSPF Passive Interfaces

• Use passive-interface default: In production networks, configure this globally under the OSPF process, then use “no passive-interface” only on interfaces where adjacencies are needed (e.g., WAN links). This enhances security by defaulting to passive on all interfaces, preventing rogue OSPF neighbors.
• Apply to LAN/stub interfaces: Always make host-facing or loopback interfaces passive to save bandwidth and CPU.
• Combine with authentication: For CCNP-level security, pair passive interfaces with OSPF MD5 or SHA authentication to mitigate risks further.
• Monitor with show commands: Regularly verify with “show ip ospf interface” to ensure no unexpected adjacencies form.
• IPv6 Consideration: For OSPFv3, the command is similar but under “router ospfv3 <process-id>”.

Configuring OSPF Passive Interfaces for IPv6 (OSPFv3)

For IPv6 networks, use OSPFv3. The passive-interface command works similarly:

Router1(config)# ipv6 router ospf 20
Router1(config-rtr)# passive-interface GigabitEthernet0/2

Or globally:
Router1(config-rtr)# passive-interface default
Router1(config-rtr)# no passive-interface GigabitEthernet0/1

Router1# show ipv6 ospf interface

Troubleshooting OSPF Passive Interfaces

• Adjacency not forming where expected: Check if the interface is accidentally passive using “show ip ospf interface <int>”. Use “no passive-interface” if needed.
• Network not advertised: Ensure the network command includes the passive interface’s subnet.
• High CPU on router: If Hellos are flooding non-passive LANs, make them passive.
• Common mistake: Forgetting “no shutdown” on interfaces—always verify with “show interfaces”.

Comparison with EIGRP Passive Interfaces

Like OSPF, EIGRP uses “passive-interface” to prevent Hellos (diffusing updates) on interfaces while advertising networks. However, EIGRP is distance-vector, so passive interfaces don’t affect LSAs but stop variance calculations on stubs. Configuration is identical under “router eigrp <AS>”.

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