📘 CCNA 200-301 v1.1
3.2 Determine how a router makes a forwarding decision by default
3.2.b — Administrative Distance (AD)
Part of: “Determine how a router makes a forwarding decision by default”
1. What is Administrative Distance (AD)?
Administrative Distance (AD) is a numerical value that routers use to decide which routing source to trust when there are multiple routes to the same destination learned from different routing sources (such as static routes, OSPF, EIGRP, RIP, etc.).
Think of it as the “trustworthiness level” or “priority number” of a route.
- Lower AD = More trustworthy
- Higher AD = Less trustworthy
Routers always choose the route with the lowest Administrative Distance if there are multiple routes to the same destination.
2. Why Administrative Distance is Important
In an enterprise network, routers can learn routes in many ways, such as:
- Manually configured static routes
- Automatically learned from routing protocols (like RIP, OSPF, or EIGRP)
- Learned from directly connected networks
If a router learns about the same destination from different sources, it must choose only one route to place in the routing table.
This is where Administrative Distance helps the router decide which one is most reliable.
3. Default Administrative Distance Values in Cisco Routers
Here is the default AD table that every CCNA student must memorize:
| Route Source | Administrative Distance | Description |
|---|---|---|
| Directly Connected | 0 | Most trusted – directly attached to the router |
| Static Route | 1 | Manually configured by the administrator |
| EIGRP (Internal) | 90 | Routes learned within the same EIGRP autonomous system |
| OSPF | 110 | Routes learned from OSPF neighbors |
| RIP | 120 | Routes learned using RIP protocol |
| EIGRP (External) | 170 | Routes learned from a different EIGRP autonomous system |
| Unknown / Unreachable | 255 | Route is not trusted – never used |
Note:
A route with an AD of 255 means “unreachable” — the router will never use that route.
4. Example in a Router Decision (IT-Focused Scenario)
Suppose a router learns the same destination network 10.10.10.0/24 in multiple ways:
| Route Source | Next Hop | AD | Metric | Result |
|---|---|---|---|---|
| Static Route | 192.168.1.1 | 1 | – | Chosen |
| OSPF | 192.168.2.1 | 110 | – | Not chosen |
| RIP | 192.168.3.1 | 120 | – | Not chosen |
Explanation:
Even though OSPF and RIP have valid paths, the static route is selected because its Administrative Distance (1) is lower than OSPF (110) and RIP (120).
The router installs only the static route in its routing table.
5. How AD Helps in a Real IT Network
In a company network, administrators often use multiple routing methods for redundancy or backup.
For example:
- Primary Route: Static route (AD = 1)
- Backup Route: OSPF route (AD = 110)
If the static route fails (e.g., the next-hop device goes down), the router automatically switches to the OSPF route, because it becomes the next best route available.
This ensures network reliability and automatic failover.
6. Adjusting Administrative Distance
Network engineers can manually change the AD of routes to control which route the router prefers.
Example:
If you want OSPF to be preferred over a static route, you can increase the AD of the static route.
Command:
Router(config)# ip route 10.10.10.0 255.255.255.0 192.168.1.1 120
Here, the static route’s AD is set to 120, which is the same as RIP.
This means if OSPF (AD = 110) also knows that network, OSPF will be preferred.
7. Key Points to Remember for CCNA Exam
- Definition:
Administrative Distance = Trustworthiness of the routing source. - Decision Rule:
Lowest AD wins → route with the lowest AD goes into the routing table. - Default AD values:
- Connected = 0
- Static = 1
- EIGRP (Internal) = 90
- OSPF = 110
- RIP = 120
- EIGRP (External) = 170
- Unreachable = 255
- You can manually adjust AD to control route preference.
- If two routes have the same AD and same destination:
Then metric (like hop count or cost) is used next to break the tie.
8. Summary
| Concept | Description |
|---|---|
| Purpose | Helps router choose the most trusted route when multiple sources exist |
| Lower AD | More preferred route |
| Used by Router | When multiple routes to the same destination are available |
| Default Values | Connected (0), Static (1), EIGRP (90), OSPF (110), RIP (120) |
| Adjustable? | Yes, manually configurable |
| 255 AD | Route is considered unusable |
✅ In Short (for quick revision)
- AD = Route Trust Level
- Lowest AD = Best route
- Static (1) usually beats Dynamic protocols
- Can be changed manually
- 255 = Never used
