Administrative Distance Explained in Routing

In networking, routers sometimes receive the same route from multiple routing protocols.
And when that happens…
the router must make an important decision: "Which source should I trust more?" 🤔
This is exactly where Administrative Distance (AD) becomes important.

What Is Administrative Distance (AD)? 💡​

Administrative Distance (AD) is a value used by routers to measure how trustworthy a route source is.
Simple rule:

✅ Lower AD = Higher Trust
❌ Higher AD = Lower Trust​

Think of AD like a trust score for routing information.
The router compares route sources and chooses the one with the lowest Administrative Distance.

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Understanding Administrative Distance Values 📊​

Basic concept:

🔥 AD = 0 → Highest Trust​

🚫 AD = 255 → Route Rejected​

The closer the value is to 0, the more confident the router is in that route source.

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Connected Route - AD = 0 🟢​

Connected routes have the highest priority.
Why?
Because the network is directly attached to the router.
No routing protocol is involved.
The router sees the connection itself.

Example:
If a router interface has an IP address configured:

interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0

That network becomes a Connected Route automatically.
Connected routes are considered the most reliable.

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Static Route - AD = 1 🔵​

A Static Route is manually configured by the network administrator.
Because an engineer explicitly defined the path, routers trust it very highly.

Example:

ip route 10.10.10.0 255.255.255.0 192.168.1.254

Static routes are commonly used for:

• Backup paths
• Small networks
• Predictable traffic routes
• Default gateways

They usually win against most dynamic routing protocols.

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EIGRP Internal - AD = 90 🟣​

EIGRP Internal Routes have an AD value of 90.
EIGRP is known for:

✅ Fast convergence
✅ Efficient routing decisions
✅ Strong performance in enterprise environments​

Internal routes are considered more trustworthy than external EIGRP routes.

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OSPF - AD = 110 🟠​

OSPF (Open Shortest Path First) is one of the most widely used enterprise routing protocols.
Features include:

✅ Open standard protocol
✅ Uses the SPF Algorithm
✅ Excellent scalability
✅ Popular in enterprise environments​

Example OSPF configuration:

router ospf 1
 network 192.168.1.0 0.0.0.255 area 0

Its default Administrative Distance = 110.

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IS-IS - AD = 115 🟡​

IS-IS is another powerful routing protocol.
You will often find it in:

• ISP environments
• Carrier networks
• Large-scale service provider infrastructure

Advantages:

✅ Stable
✅ Scalable
✅ High-performance routing​

Default AD: 115

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RIP - AD = 120 🔴​

RIP (Routing Information Protocol) is one of the oldest routing protocols.
Characteristics:

❌ Slower convergence
❌ Limited scalability
❌ Maximum Hop Count = 15​

Because of these limitations, RIP is rarely used in modern enterprise networks.

Example:

router rip
version 2
network 192.168.1.0

Default AD: 120

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EIGRP External - AD = 170 🟤​

External EIGRP routes come from outside the original EIGRP autonomous system.
Because they originate externally, routers assign them a higher Administrative Distance.
Default value: 170
This makes them less trusted than EIGRP Internal Routes.

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BGP - AD = 200 ⚫🌍​

BGP (Border Gateway Protocol) is the protocol that powers the internet.
Seriously.
It handles routing between:

• ISPs
• Countries
• Large organizations
• Internet backbone networks

BGP is critical for global connectivity.
Yet its default AD is: 200
Why?
Because BGP primarily deals with external routing information.

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Unknown Routes - AD = 255 ❌​

When AD reaches 255: the router completely rejects the route.
Result: 🚫 The route will never enter the routing table.

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How Does the Router Choose? 🧠​

Imagine the router learns the same network from two protocols:

OSPF → AD 110​

RIP → AD 120​

Which one wins?

✅ OSPF
Because: Lower Administrative Distance = Higher Trust
The router always prefers the route source with the smaller AD value.

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Administrative Distance Quick Reference Table 📋​

Route Source	Default AD
Connected	0
Static Route	1
EIGRP Internal	90
OSPF	110
IS-IS	115
RIP	120
EIGRP External	170
BGP	200
Unknown	255

Why Administrative Distance Matters in Real Networks 🚀​

Understanding AD is essential for:

✅ Network Troubleshooting
✅ Route Selection Analysis
✅ Multi-Protocol Environments
✅ Backup Route Design
✅ Enterprise Routing Management​

Without understanding Administrative Distance, routing behavior can look confusing very quickly.

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Final Thoughts 🔥​

Administrative Distance is basically the router’s way of asking: "Which routing source do I trust the most?"
The rule is simple: Lower Number = Higher Trust

Understanding AD helps network engineers predict:

• Which routes win
• Why routers make certain decisions
• How routing protocols interact inside real infrastructures

Because in networking…
routing is not only about finding paths - it's about deciding which information deserves trust. 🌐⚡