Lesson 34: Administrative Distance (AD) and Metrics
Administrative Distance (AD)
AD is the measure of the trustworthiness of the source of the route information. It is a value between 0 and 255. Lower is better.
| Route Source | Default AD |
|---|---|
| Connected Interface | 0 |
| Static Route (Next-Hop) | 1 |
| EIGRP Summary Route | 5 |
| External BGP | 20 |
| Internal EIGRP | 90 |
| OSPF | 110 |
| RIP | 120 |
| External EIGRP | 170 |
| Unknown (Unusable) | 255 |
If a router learns the same destination network via OSPF (AD 110) and EIGRP (AD 90), it installs the EIGRP route because 90 is lower than 110.
Metrics
Metrics are used only when comparing multiple routes learned via the same routing protocol. Examples:
- OSPF: Uses Cost (based on link bandwidth).
- RIP: Uses Hop Count (number of routers to destination).
- EIGRP: Uses a complex composite metric (Bandwidth, Delay, Reliability, Load).