Administrative distance is what is used to select the best path when a router has two different paths to the same destination via two different routing protocols.
- Most routing protocols are not compatible with other protocols.
- In a network with multiple routing protocols, being able to select the best path between multiple protocols is critical.
- Administrative distance defines the reliability of a routing protocol.
- Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.
- IPv6 uses the same distances as IPv4.
- Default Administrative Distances:
See CCNA 3.3.a: Admin Distance for a full list