Administrative Distance Explained: Managing Routing Protocol Priorities

🌐 Administrative Distance (AD) – The Hidden Decision Maker in Routing

Imagine multiple people giving you directions to the same place—but who do you trust more?

That’s exactly what Administrative Distance (AD) does in networking.

It helps routers decide which route source is more reliable.

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📚 Table of Contents

• What is Administrative Distance?
• Math Behind AD (Simple)
• RIP Configuration
• EIGRP Configuration
• OSPF Configuration
• BGP Configuration
• CLI Outputs
• Modern Improvements
• Best Practices
• Key Takeaways

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🧠 What is Administrative Distance?

Administrative Distance is a value that represents trust.

Lower value = more trusted route

Basic Rule:

\[ Best\ Route = \min(AD) \]

👉 Router always selects the route with the lowest AD.

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📐 Simple Math Logic

Let’s say we have three routes:

Protocol	AD
RIP	120
OSPF	110
EIGRP	90

\[ Selected = \min(120, 110, 90) = 90 \]

✅ EIGRP wins.

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📡 RIP Configuration

Router(config)#router rip Router(config-router)#network 192.168.15.0 Router(config-router)#distance 15 192.168.15.1 0.0.0.0 Router(config-router)#distance 200 192.168.15.0 0.0.0.255 Router(config-router)#distance 255

💡 Explanation

Different AD values are assigned based on source IP and network range.

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⚡ EIGRP Configuration

Router(config)#router eigrp 111 Router(config-router)#network 192.168.16.0 Router(config-router)#distance eigrp 55 200

💡 Explanation

First value = internal routes, second = external routes.

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🔄 OSPF Configuration

Router(config)#router ospf 66 Router(config-router)#distance ospf intra-area 105 Router(config-router)#distance ospf inter-area 115 Router(config-router)#distance ospf external 125

💡 Explanation

• Intra-area → same area
• Inter-area → different area
• External → outside OSPF

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🌍 BGP Configuration

Router(config)#router bgp 65520 Router(config-router)#distance bgp 115 220 50

💡 Explanation

• External BGP
• Internal BGP
• Local routes

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🖥️ CLI Output

Click to Expand

Router#show ip route

R 192.168.1.0 [120/1]
O 192.168.1.0 [110/2]
D 192.168.1.0 [90/3]

Selected Route: EIGRP (AD 90) 

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🚀 Modern Improvements

• Support for prefix-lists
• Route-map integration
• Better debugging tools
• Granular AD control

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🛠️ Best Practices

• Always test before deployment
• Use specific filters instead of global changes
• Avoid setting AD too low unless necessary
• Monitor routing behavior after changes

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💡 Key Takeaways

• AD decides trust between routing protocols
• Lower AD = higher priority
• Math helps predict routing decisions
• Modern tools allow precise control

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🎯 Final Thought

Administrative Distance is not just a number—it’s a decision-making system.

Master it, and you control how your network thinks.