Administrative Distance (AD)
Administrative Distance (AD) is a value used to decide which source to trust when multiple sources of route information (static, various dynamic routing protocols, etc.) exist for the same destination. A lower value means higher priority, and that route is the one adopted into the routing table.
Administrative distance is used only to compare routes learned from different sources. The priority between routes learned from the same source (e.g., OSPF) is instead determined by the metric. See also Routing.
Default Values by Route Type
The typical route types on Cisco IOS(-XE) and their default administrative distances are as follows.
| Route Type | Default Administrative Distance |
|---|---|
| Connected | 0 |
| Static | 1 |
| eBGP | 20 |
| EIGRP (internal) | 90 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EIGRP (external) | 170 |
| iBGP | 200 |
The default AD for a static route, configured as described in Static Routing, is 1, the second-highest priority after a directly connected route (0). This means that when route information for the same destination is available from both a static route and a dynamic routing protocol, the static route is preferred by default.
One application of administrative distance is the floating static route, a static route deliberately configured with a high AD so it can serve as a backup route. See Floating Static Route for details.
References
| RFC | Title | Overview |
|---|---|---|
| RFC 1812 | Requirements for IP Version 4 Routers | Defines requirements for IPv4 routers, including concepts related to route selection priority. |