Administrative Distance

Introduction

In computer networking, routing is the process of forwarding data packets from one network to another. Routing tables play a vital role in this process by helping routers determine the best path for forwarding data. However, not all routes are equal, and some are more reliable than others. This is where administrative distance comes in.

What is Administrative Distance?

Administrative distance (AD) is a value that is used to rank the reliability of routes in a routing table. It is a measure of the trustworthiness of a routing protocol, and it is used by routers to determine which routes are more trustworthy and should be preferred over others.

How is Administrative Distance Used?

Administrative distance is used to select the best route among multiple routes for the same destination network. When a router receives a packet, it looks at the destination IP address and checks its routing table to determine the best path for forwarding the packet. If there are multiple paths available, the router selects the one with the lowest administrative distance.

How is Administrative Distance Calculated?

The administrative distance is a value assigned to each routing protocol by the network administrator or the router manufacturer. It is a static value that can be changed dynamically. The lower the administrative distance value, the more reliable the routing protocol.

Here are some common administrative distance values for popular routing protocols:

Directly connected interfaces: 0

Static routes: 1

Border Gateway Protocol (External - eBGP): 20

Enhanced Interior Gateway Routing Protocol (EIGRP): 90

Open Shortest Path First (OSPF): 110

Border Gateway Protocol (External - iBGP): 200

Comparing Different Administrative Distances

In a routing table, entries with different administrative distances are prioritized based on their value. When a router has multiple routes for the same destination network, it selects the route with the lowest administrative distance value.

For example, let's say we have the following routing table:

Destination Subnet Mask   Next Hop Administrative Distance

10.0.0.0         255.0.0.0         192.168.1.1   1

10.0.0.0         255.0.0.0         192.168.1.2   2

10.0.0.0         255.0.0.0         192.168.1.3   3

In this example, all three entries have the same destination network and subnet mask. However, they have different Next Hop addresses and administrative distances. When a router receives a packet destined for the 10.0.0.0 network, it will select the route with the lowest administrative distance. In this case, it will choose the first route with an AD of 1.

Route Priorities for Different Administrative Distance Values

In a routing table, entries with the same network mask length and administrative distance value are prioritized based on the metric value, as mentioned earlier. However, network administrators must also consider the reliability and performance of the routing protocol when assigning priorities. For example, if there are two routing protocols with the same administrative distance and metric value, but one is more reliable and has better performance, the network administrator may assign it a higher priority.

Conclusion

Administrative distance is an essential component of routing tables that helps routers determine the best path for forwarding data. Network administrators must consider factors such as the reliability and performance of routing protocols when assigning administrative distance values and assigning priorities to different routes. By prioritizing routes based on administrative distance and metric values, network administrators can ensure efficient data transmission and effectively manage their networks.