Administrative distance in the context of network routing is a metric that routers use to select the best route when there are two or more different routes to the same destination from two different routing sources. It is a key concept in IP routing and helps maintain the consistency and reliability of routing information.

Administrative distance is essentially a measure of the credibility or reliability of the source of the route. Each routing protocol (such as OSPF, EIGRP, BGP, etc.) is assigned an administrative distance value predetermined by the network device manufacturer. These values ​​can be modified by network administrators to influence route selection.

Administrative distance values ​​range on a scale typically from 0 to 255, with a lower value indicating a more reliable source. For example, a route learned directly through a connected interface has an administrative distance of 0, making it more reliable than any other route learned through routing protocols. Here are some common administrative distance default values ​​for various routing protocols:

• Directly connected: 0
• Static: 1
• EIGRP (internal): 90
• OSPF: 110
• RIP: 120
• EIGRP (external): 170
• BGP (external): 20
• BGP (internal): 200

When evaluating multiple routes to the same destination, the router uses administrative distance to determine which of them is considered the most reliable and, therefore, which should be placed in the routing table as the preferred route. If the administrative distance is the same for two routes, the router will use protocol-specific metrics to decide which route is best.