Understanding Administrative Distance

 Administrative Distance

Administrative distance (AD) is a numerical value assigned to different routing protocols that determine which route a router will choose when multiple paths are available to the same destination, essentially acting as a trust level for the source of routing information, with a lower AD value signifying a more trusted source and thus being preferred by the router; essentially, when a router receives routes from multiple protocols for the same destination, the route with the lowest AD will be chosen to forward traffic.

Key points about administrative distance:

• Trust level: AD's primary function is to indicate the trustworthiness of a routing protocol, with a lower value representing a more reliable source of routing information.
• Router decision-making: When a router receives routes for the same destination from multiple protocols, it compares their AD values and selects the route with the lowest AD to install in its routing table.
• Configuration: Most network devices, like Cisco routers, have a default AD value for each routing protocol, but network administrators can manually adjust these values to prioritize specific routes based on their network design.
• Range: Administrative distance values typically range from 0 (most trusted) to 255 (least trusted).

Example scenario:

• Imagine a network where both OSPF and RIP are running.
• On a Cisco router, OSPF usually has a default AD of 110, while RIP has a default AD of 120.
• If the router receives routes to the same destination from both protocols, it will choose the OSPF route because it has a lower AD, indicating a more reliable path.

Administrative Distance chart:

Important considerations:

Static routes: Static routes often have a very low AD, making them a good choice for critical connections where you want to force traffic along a specific path.

Directly connected networks: Directly connected networks usually have the lowest AD value and are considered the most reliable.

Impact on network design: Properly configuring AD values is crucial for network stability and performance, as it allows administrators to control traffic flow based on the trustworthiness of different routing sources.

This post is covered in Network+.