2.1 Explain characteristics of routing technologies
Route Selection
📘CompTIA Network+ (N10-009)
Administrative Distance (AD)
1. Definition
- Administrative Distance (AD) is a value that routers use to decide which routing information to trust when multiple routes to the same destination exist.
- Think of it as a trust score: lower AD values mean the route is more trusted, and higher values mean the route is less trusted.
- AD is not the same as a metric. Metrics measure the best path within a routing protocol, while AD compares different routing protocols against each other.
2. Why Administrative Distance Matters
In a network, a router can learn about the same destination network from different sources, for example:
- Directly connected networks
- Static routes
- Dynamic routing protocols (RIP, OSPF, EIGRP, BGP, etc.)
When a router receives multiple routes to the same destination from different sources, it needs a way to choose which route to put in the routing table. That’s where AD comes in.
Example:
- A network 192.168.1.0/24 is reachable via:
- OSPF
- EIGRP
- Router checks the AD:
- OSPF AD = 110
- EIGRP AD = 90
- Router chooses EIGRP because 90 < 110 → EIGRP is more trusted.
3. Default Administrative Distance Values
CompTIA Network+ exam expects you to know the default AD values of common routing sources. Here’s a simplified table:
| Routing Source | Default AD | Explanation |
|---|---|---|
| Directly connected network | 0 | Most trusted – directly connected to the router’s interface |
| Static route | 1 | Very trusted – manually configured by the network admin |
| EIGRP (internal) | 90 | Trusted dynamic routing protocol within the same autonomous system |
| OSPF | 110 | Less trusted than EIGRP but still a standard dynamic protocol |
| RIP | 120 | Older, less efficient protocol → less trusted |
| External EIGRP | 170 | External routes from outside the EIGRP AS |
| Unknown/Untrusted | 255 | Cannot be used – router ignores this route |
Key point: Lower number = more trusted route.
4. How AD Works in Routing
When multiple routes exist:
- Router checks AD of each route.
- Route with the lowest AD is chosen for the routing table.
- Other routes are ignored (but can be kept as backup in some cases).
Example Scenario:
- Router has three routes to 10.10.10.0/24:
- Directly connected: AD = 0
- Static: AD = 1
- OSPF: AD = 110
- Route selected: Directly connected (AD 0)
- Why: Most trusted route always wins, even if OSPF reports a “shorter path.”
5. Practical IT Examples
- Directly connected: A router’s interface has 192.168.1.1/24 → AD = 0 → always preferred.
- Static route: Admin configures
ip route 172.16.0.0 255.255.0.0 192.168.1.2→ AD = 1 → highly trusted. - Dynamic routes: Router learns about 10.0.0.0/8 from OSPF (AD = 110) and RIP (AD = 120) → OSPF route preferred.
- Backup routes: If the primary route fails, the router may use a route with a higher AD as a backup.
6. Key Exam Takeaways
For CompTIA Network+:
- AD is a trust value used by routers to choose the best route.
- Lower AD = higher trust.
- Directly connected and static routes have the lowest AD.
- Dynamic routing protocols have higher AD values, which differ by protocol.
- AD determines which route goes into the routing table, not the path cost or metric.
- AD 255 → route is unusable.
