Administrative Distance
Administrative Distance (AD) is a value assigned to a routing protocol that indicates the trustworthiness or reliability of the source of routing information. Lower AD values indicate a more trusted source.
Administrative Distance
Administrative Distance (AD) is a value assigned to a routing protocol that indicates the trustworthiness or reliability of the source of routing information. Lower AD values indicate a more trusted source.
How Does Administrative Distance Work?
In a network where multiple routing protocols might be advertising routes to the same destination, routers use AD to decide which route to install in the routing table. If a router learns about a destination from two different sources (e.g., one from OSPF and one from EIGRP), it compares their AD values. The protocol with the lower AD is considered more reliable, and its route will be preferred. Each routing protocol has a default AD value, but these can often be manually adjusted.
Comparative Analysis
Administrative Distance is a Cisco proprietary concept, though similar mechanisms exist in other vendor implementations. It provides a simple yet effective way to manage routing information from diverse sources. Compared to metrics used within a single routing protocol (like hop count or bandwidth), AD operates at a higher level, prioritizing the source of the information itself before considering the path cost.
Real-World Industry Applications
AD is fundamental in enterprise and service provider networks that often run multiple routing protocols simultaneously (e.g., static routes, OSPF, EIGRP, BGP). It allows network administrators to control how routes are selected, ensuring that critical routes (like static routes to a default gateway) are always preferred over dynamically learned routes, or that one dynamic protocol is favored over another.
Future Outlook & Challenges
The concept of AD remains relevant as networks grow in complexity. Challenges include understanding the default AD values for various protocols and correctly configuring custom AD values to achieve desired routing behavior. Misconfigurations can lead to suboptimal routing paths or network instability.
Frequently Asked Questions
- What is Administrative Distance? A value indicating the trustworthiness of a routing source.
- Which is preferred: a lower or higher AD value? Lower AD values are preferred, indicating a more trusted source.
- What happens if a router learns the same route from two protocols with different ADs? The route with the lower AD is installed in the routing table.