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Configure OSPF Clear-Text Authentication

Learn to configure, verify, and troubleshoot Cisco IOS OSPF clear-text authentication between neighboring routers.

OSPF clear-text authentication adds a shared password to OSPF packets exchanged on an interface. This helps ensure that only routers with the expected authentication settings participate in the OSPF domain. This lesson assumes you understand basic IPv4 addressing, Cisco IOS command modes, OSPFv2 interfaces, areas, and neighbor states.

For a related reference, see OSPF clear-text authentication.

Purpose of OSPF Authentication

OSPF, or Open Shortest Path First, is a link-state interior gateway routing protocol. OSPF routers exchange control packets, including Hello packets, to discover and maintain neighbors. After eligible neighbors agree on required parameters, they form an OSPF adjacency and synchronize link-state information.

OSPF authentication validates routing protocol packets received from a neighboring interface. It helps prevent an unauthorized device from becoming an OSPF neighbor or injecting incorrect routing information into the routing domain.

When authentication is enabled on an OSPF segment, both routers must successfully authenticate the packets they receive. Authentication is therefore part of adjacency formation and maintenance. If authentication fails, Hello packets are rejected, and the relationship cannot reach or remain in the Full state.

OSPF Authentication Methods

Traditional OSPFv2 deployments commonly describe three choices: no authentication, simple password authentication, and MD5 authentication.

Method	Authentication Type	Password Protection	Typical Use	Security Consideration
No authentication	None	No password	Unprotected labs or networks where authentication is not required	Any device able to access the segment may attempt to participate in OSPF
Clear-text or simple password authentication	Simple password, traditionally OSPF authentication type 1	Password is carried without cryptographic protection	Legacy environments, labs, and training exercises	Not suitable where packet capture must not reveal the password
MD5 authentication	Cryptographic message-digest authentication, traditionally OSPF authentication type 2	Uses a keyed digest rather than sending the password as readable data	Environments requiring stronger protection and supported by the platform	Stronger than simple authentication, but it is still routing-protocol authentication rather than user-data encryption

Clear-text authentication is the simple password-based OSPF method. The configured authentication key is included in the OSPF authentication data without cryptographic protection. Someone who can capture the relevant OSPF packets may be able to read the password.

Use clear-text authentication only when a legacy, lab, or training requirement justifies it. For stronger protection, use a supported method such as MD5 according to the platform capabilities and the network security design.

How Clear-Text Authentication Works

On Cisco IOS, simple OSPF authentication has two interface-level parts:

ip ospf authentication-key secret configures the authentication key.
ip ospf authentication enables simple authentication on that interface.

Both commands matter. Configuring a key without enabling interface authentication does not produce the intended result. Enabling authentication without a matching key on the peer produces an authentication mismatch.

Every router that must become a neighbor on the same relevant OSPF segment needs the same authentication mode and the same password. If one router uses simple authentication while the other uses no authentication, or if the two passwords differ, the routers reject each other's authenticated Hello packets.

On a multiaccess network, such as Ethernet, apply the configuration consistently to every OSPF router that should form a relationship on that segment. A DR, or Designated Router, is elected on a multiaccess OSPF network to reduce the number of required adjacencies. A BDR, or Backup Designated Router, is the standby for the DR role. Authentication must still be consistent for all intended OSPF participants on the segment.

Two-Router Topology

Assume R1 and R2 are directly connected through their Ethernet interfaces. OSPF is already enabled on the link, both interfaces are in the same OSPF area, and the routers are intended to become neighbors.

R1 uses GigabitEthernet0/0 for the connection.
R2 uses GigabitEthernet0/0 for the connection.
The shared simple authentication password is secret.
The OSPF area and other neighbor parameters already match.

The password is a conceptual lab value. In a real environment, select credentials according to the organization's security policy and protect access to device configuration.

Configure Clear-Text Authentication on Cisco IOS

Router 1

R1> enable
R1# configure terminal
R1(config)# interface GigabitEthernet0/0
R1(config-if)# ip ospf authentication-key secret
R1(config-if)# ip ospf authentication
R1(config-if)# end

Router 2

R2> enable
R2# configure terminal
R2(config)# interface GigabitEthernet0/0
R2(config-if)# ip ospf authentication-key secret
R2(config-if)# ip ospf authentication
R2(config-if)# end

The interface name is only an example. Replace it with the interface that actually connects to the OSPF neighbor. The interface must already participate in OSPF, either through the broader OSPF network configuration or through interface-level OSPF commands.

Authentication can be configured before an adjacency forms or while an adjacency is already active. When the setting changes, the existing relationship may reset while both routers renegotiate using the new authentication requirements. Configure both ends consistently to minimize disruption.

Configuration Checklist

Requirement	Router 1	Router 2	Must Match?
OSPF enabled on the connecting interface	Yes	Yes	Both interfaces must participate in OSPF
OSPF area	Intended area	Same intended area	Yes
Authentication mode	Simple authentication enabled	Simple authentication enabled	Yes
Authentication password	`secret`	`secret`	Yes, including exact characters
Interface operational state	Up/up	Up/up	Both interfaces must be usable

Verify OSPF Authentication

Inspect the OSPF Interface

Use the OSPF interface command to inspect the settings applied to the OSPF-facing interface:

R1# show ip ospf interface GigabitEthernet0/0

Review the output for an indication that simple authentication is enabled. Cisco IOS output varies by release, but the interface details should identify simple authentication rather than showing no authentication or a different method.

Run the same command on R2:

R2# show ip ospf interface GigabitEthernet0/0

Check the Neighbor State

R1# show ip ospf neighbor

The expected peer should appear in the neighbor table. On a point-to-point link, a healthy adjacency commonly reaches FULL. On a broadcast Ethernet segment, the state and relationship details also reflect DR and BDR roles. The exact displayed state can vary by router role and network type, but an intended fully synchronized relationship should not remain stuck in an early state.

Inspect the Interface Configuration

R1# show running-config interface GigabitEthernet0/0

Confirm that the interface configuration contains both the authentication key and the command that enables simple authentication:

interface GigabitEthernet0/0
 ip ospf authentication-key secret
 ip ospf authentication

Repeat the check on R2. Be aware that configuration display and password handling can vary with device security settings. Restrict access to running configuration output because it may expose the clear-text key.

Verification sequence: First verify the local interface configuration, then compare the same interface on the peer, and finally check show ip ospf neighbor for the resulting relationship.

Authentication Mismatch Examples

One Router Uses Authentication and the Other Does Not

Suppose R1 contains both authentication commands, but R2 has neither command. R1 expects authenticated OSPF packets, while R2 sends packets without the required authentication. The Hello packets fail validation, so the neighbor does not become fully adjacent.

The Passwords Differ

Suppose R1 uses secret and R2 uses Secret. Password comparison is exact; capitalization matters. The authentication type may match, but the authentication keys do not, so the routers reject the packets.

The Configuration Is on the Wrong Interface

Authentication applies to the interface where the commands are configured. Applying the commands to a management interface or another OSPF interface does not authenticate packets received on the actual link to the neighbor. Select the interface shown by the OSPF interface and neighbor information.

Troubleshooting Guide

Symptom	Likely Cause	Verification Command	Corrective Action
No OSPF neighbor appears	Authentication mismatch, wrong interface, or another OSPF parameter mismatch	`show ip ospf interface GigabitEthernet0/0` `show ip ospf neighbor`	Compare both interfaces and correct authentication, addressing, area, and neighbor parameters
Authentication-related log message	Hello packets are rejected because authentication type or key is incorrect	`show ip ospf interface GigabitEthernet0/0` `show running-config interface GigabitEthernet0/0`	Use the same authentication mode and exact key on both peer interfaces
Authentication is enabled on only one peer	The other peer uses no authentication	`show ip ospf interface` on both routers	Enable simple authentication and configure the matching key on the other OSPF interface, or remove it consistently from both sides
Different passwords are configured	The authentication keys do not match, including capitalization differences	`show running-config interface` on both routers	Replace the key on one side so both peers use the identical value
Authentication is correct but adjacency still fails	Area, subnet, timers, network type, MTU, passive state, interface state, or OSPF participation differs	`show ip ospf interface` `show ip interface brief` `show ip ospf neighbor`	Correct the non-authentication OSPF mismatch and verify the neighbor again

Systematic Troubleshooting Procedure

Confirm the physical and data-link connection. Use show ip interface brief and verify that both interfaces are operational.
Confirm that the selected interfaces have IP addresses in the expected subnet for the link.
Run show ip ospf interface on both routers. Compare the area, network type, Hello interval, dead interval, passive status, and authentication information.
Confirm that simple authentication is enabled on both matching interfaces.
Compare the authentication keys. They must be identical, including letter case and punctuation.
Confirm that each interface is actually included in the intended OSPF process and area.
Check show ip ospf neighbor to determine whether the neighbor is absent, stuck in an intermediate state, or fully adjacent.
If authentication is correct, investigate other common causes such as an area ID mismatch, different Hello or dead timers, different network types, an MTU mismatch, or a passive interface.

Operational Cautions and Security Limitations

The simple password is transmitted in readable form in the OSPF packet authentication field. It does not provide strong protection against packet capture.
OSPF authentication validates routing protocol packets; it does not encrypt user data, application traffic, or the complete routing exchange.
A clear-text key visible in device configuration output can be disclosed to anyone who has sufficient configuration access.
Use stronger supported OSPF authentication, such as MD5 in traditional OSPFv2 designs, when the environment requires better protection.
When changing authentication on a live link, expect the adjacency to reset temporarily. Coordinate changes on both ends and verify recovery afterward.

Removing Clear-Text Authentication

To remove interface-level simple authentication, enter interface configuration mode and remove both settings:

R1# configure terminal
R1(config)# interface GigabitEthernet0/0
R1(config-if)# no ip ospf authentication
R1(config-if)# no ip ospf authentication-key
R1(config-if)# end

Do not remove authentication on only one side while leaving it enabled on the peer. That creates the same mismatch as an incorrectly configured link. If authentication is no longer required, remove it consistently from all intended peers. If authentication is required, configure an appropriate method consistently instead.

Exam-Relevant Notes

Summary

To configure Cisco IOS OSPF clear-text authentication, enter the OSPF-facing interface, configure an authentication key, and enable simple authentication. Apply the same mode and exact password to every intended neighbor on the segment. Verify the interface with show ip ospf interface, verify the configuration with show running-config interface, and verify the relationship with show ip ospf neighbor. Because the password is exposed without cryptographic protection, reserve this method for appropriate legacy, lab, or training situations and prefer a stronger supported method when security requirements demand it.