What is Administrative Distance

In Computer Networking, when it comes to the best route selection, which is also called the best path selection, different attributes are determined by the routers to select the best path and send the packets. These attributes are route specificity, Metric, and Administrative Distance. Each of these attributes is considered in a different scenario. For example, the metric is used to determine the best route over multiple paths learned via one single protocol (same protocol). Let’s say if a router finds three paths to a destination in its routing table and all these three routes are advertised using Routing Information Protocol (RIP), then the router uses the metric to determine the best path.

The administrative distance is used in a different scenario than that, which we are going to discuss in this post. 

Introduction to Administrative Distance

From above, we learned that the metric structure and algorithm are used by routers to determine the best path if the paths are learned through the same protocol. But what if the routers have to select the best path over multiple routes learned by different routing protocols? Is metric compatible in that case as well? The answer is, No, it is NOT.

The ability to share route information and pick the optimal path across various routing protocols is crucial in a network where several routing protocols are present.

When routers learn multiple routes to the same destination advertised through different routing protocols, they prefer the administrative distance to determine the best path. Administrative distance rates the trustworthiness of routing information and defines how much a routing protocol is reliable.

It is a numeric integer value ranging from 0 to 255 used to prioritize each routing protocol's reliability from the most reliable routing protocol to the less reliable and trustable routing protocol. The routing protocol with a smaller administrative distance value is the most trustable and reliable. Therefore, any routing protocol with the administrative distance value of 0 is considered to be the most trustable and reliable, while with the administrative distance value of 255 it’s considered to be the worst.

We have already said that the protocol is considered to be more reliable if its administrative distance value is smaller. For example, if a router learns about a route to a network advertised both through Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) the router prefer to select Interior Gateway Routing Protocol (IGRP) because the rip administrative distance is 120 while the EIGRP administrative distance is 100. and that’s why it’s considered to be more reliable than RIP. However, if for any reason the route learned by EIGRP is down, the router will use the RIP until EIGRP is up and running again.

It is also very common that sometimes the route selected based on its lower administrative distance value would have less bandwidth or connections speed than the route ignored, because of having a higher administrative distance value. in such cases, the network administrators can change the default administrative distance values of a route to change the ranking manually and chose the best path they prefer packets should be forwarded through. On the Cisco routers, the default Administrative distance value of static route is 1 which makes it a preferred route over the routes learned by dynamic routing protocols.

Keep in mind that the significance of Administrative Distance is local only and it is not advertised on the routing updates.

The list of Default Administrative Distance Values

In the following table, you can find the list of all cisco supported protocols with their default administrative distance values:

Routing Protocol	Default Administrative Distance Value
Directly connected interface	0
Static route	1
Enhanced Interior Gateway Routing Protocol summary route - EIGRP   administrative distance	5
External Border Gateway Protocol (BGP)  - external BGP administrative distance	20
Internal EIGRP	90
IGRP Administrative Distance	100
OSPF administrative distance	110
Intermediate System-to-Intermediate System (IS-IS) Administrative Distance	115
Routing Information Protocol (RIP administrative distance)	120
Exterior Gateway Protocol (EGP) administrative distance	140
External EIGRP	170
Internal BGP administrative distance	200
Unknown	255

Please keep in mind that the router considers 255 as the worst administrative distance and does not recognize it and will not allow it into the routing table.