Cisco Administrative distance
What is an Administrative distance
The administrative distance (AD) value is assigned by the router on a per-protocol basis. Routers, by design, should not install multiple routes into the routing table as this has the potential to cause routing loops.
While a router may run multiple routing protocols on the same device, it is necessary for the router to implement a process to ensure that multiple routes, pointing to the same destination do not simultaneously exist in the routing table.
Each process running on a router advertises its administrative distance value to the local router. The router uses this value to determine which route should be used.