Administrative Distance (AD) is a concept in networking — specifically in routing protocols — that determines the trustworthiness or priority of a route when multiple routing sources offer paths to the same destination.
🧠 What Is Administrative Distance?
It’s a numerical value (from 0 to 255) that routers use to rank routes from different sources.
-
Lower AD = Higher Trust
-
If multiple routes exist to the same destination, the router chooses the one with the lowest AD.
📊 Common Administrative Distance Values (Cisco Defaults)
| Route Source | AD Value |
|---|---|
| Directly connected | 0 |
| Static route | 1 |
| EIGRP (internal) | 90 |
| OSPF | 110 |
| RIP | 120 |
| External EIGRP | 170 |
| Unknown / Untrusted | 255 (never used) |
📦 Example Scenario
Let’s say a router learns about the same destination network 10.0.0.0/24 from:
-
OSPF (AD 110)
-
EIGRP (AD 90)
Even if the OSPF route has a shorter metric (e.g., fewer hops), the router will prefer the EIGRP route because it has a lower administrative distance.
🎯 Why It Matters
-
Prevents routing conflicts: Helps routers resolve which route to install in the routing table.
-
Useful in fallback scenarios: You can configure a backup static route with a higher AD so it only takes over if the preferred route fails.
-
Ensures stability: Avoids route flapping between protocols of different trust levels.
🔧 Can You Change AD?
Yes. Most routers allow you to manually adjust the administrative distance:
-
To prefer one protocol over another even if it’s not the default.
-
For policy-based routing, traffic engineering, or backup routing.
✅ Summary
| Term | Definition |
|---|---|
| Administrative Distance | A numeric value representing the trust level of a routing source. |
| Lower AD | Means more trusted, gets installed in the routing table over higher AD routes. |
| Purpose | Helps the router choose the best route when multiple protocols suggest routes to the same destination. |