OSPF Authentication Configuration (Plain Text and MD5)

Authentication Types

1. Plain Text (Type 1)

• Password sent in clear text
• ❌ Not secure (visible in packet captures)
• Use only in lab environments

2. MD5 (Type 2)

• Password hashed with MD5
• ✅ More secure (password not visible)
• ✅ Recommended for production

3. SHA (Newer IOS)

• Available in newer IOS versions
• Even more secure than MD5

Plain Text Authentication

Interface-Level Configuration

interface gi0/0
 ip ospf authentication
 ip ospf authentication-key MyPassword123

What this does:

• Enables authentication on this interface
• Sets password to "MyPassword123"
• Sent in clear text with OSPF packets

Area-Level Configuration

router ospf 1
 area 0 authentication

interface gi0/0
 ip ospf authentication-key MyPassword123

What this does:

• Enables authentication for all interfaces in Area 0
• Each interface still needs password configured

MD5 Authentication (Recommended)

Interface-Level Configuration

interface gi0/0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 SecurePassword456

Key components:

• message-digest = MD5 authentication
• 1 = Key ID (must match on both routers)
• md5 = Hash type
• SecurePassword456 = Actual password

Area-Level Configuration

router ospf 1
 area 0 authentication message-digest

interface gi0/0
 ip ospf message-digest-key 1 md5 SecurePassword456

Complete Configuration Example

R1 Configuration (MD5)

interface gi0/0
 description Link to R2
 ip address 10.1.1.1 255.255.255.252
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 MySecretPassword

router ospf 1
 router-id 1.1.1.1
 network 10.1.1.0 0.0.0.3 area 0

R2 Configuration (MD5 - Must Match!)

interface gi0/0
 description Link to R1
 ip address 10.1.1.2 255.255.255.252
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 MySecretPassword  ← Same password

router ospf 1
 router-id 2.2.2.2
 network 10.1.1.0 0.0.0.3 area 0

Verification

Check Authentication Status

R1# show ip ospf interface gi0/0 | include auth
  Simple password authentication enabled

or (for MD5):

R1# show ip ospf interface gi0/0 | include auth
  Message digest authentication enabled
    Youngest key id is 1

Check Neighbor Adjacency

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2         1     FULL/  -        00:00:35    10.1.1.2        Gi0/0

✅ FULL state = Authentication successful

Changing Authentication Keys (Key Rollover)

Problem: Changing password breaks adjacencies

Solution: Key rollover (configure multiple keys)

Step 1: Add New Key (Both Routers)

R1(config-if)# ip ospf message-digest-key 2 md5 NewPassword
R2(config-if)# ip ospf message-digest-key 2 md5 NewPassword

Now both keys (1 and 2) are active

Step 2: Remove Old Key (Both Routers)

R1(config-if)# no ip ospf message-digest-key 1
R2(config-if)# no ip ospf message-digest-key 1

Adjacency never drops during this process

Troubleshooting Authentication Issues

Symptom: Neighbors Not Forming

Check authentication mismatch:

R1# debug ip ospf adj
*Mar 18 01:15:23: OSPF-1 ADJ Gi0/0: Rcv pkt from 10.1.1.2, area 0.0.0.0 : mismatched authentication type

Common causes:

• One router has auth enabled, other doesn't
• Different auth types (plain text vs MD5)
• Different passwords
• Different key IDs

Verify Authentication Config

R1# show run interface gi0/0 | include authentication
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 7 060506324F41584B56

Note: Password shown encrypted (type 7)

Check Packet Captures

Without auth: OSPF packets visible in plain text
With MD5: Authentication field populated, password hashed

Best Practices

1. Always Use MD5 (or SHA) in Production

interface gi0/0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 StrongPassword123!

2. Use Strong Passwords

• ✅ 12+ characters
• ✅ Mix of letters, numbers, symbols
• ❌ Avoid "cisco", "password", etc.

3. Use Key Rollover for Password Changes

Never remove old key before adding new one

4. Document Key IDs

Maintain a spreadsheet:

5. Encrypt Passwords in Config

service password-encryption

Result: Passwords stored as type 7 (weak encryption, but better than plain text)

Summary

Now you know:

✅ Why authenticate OSPF — Prevent rogue routers
✅ Plain text vs MD5 — Use MD5 in production
✅ Interface vs area-level — Both methods work
✅ How to configure — message-digest-key command
✅ Key rollover — Change passwords without downtime
✅ Troubleshooting — Debug and verify commands

Next Step:
Authentication secures OSPF. For scalability, learn about OSPF Stub Areas next.

Internal Links:

• ← Multi-Area OSPF (Article 12)
• → OSPF Stub Areas (Article 14)
• → Authentication Mismatch Troubleshooting (Article 21)