This article is all about Some important known facts about the Administrative distance which is used by routers to choose a path for sending a packet from one router to another router. Many of the routing protocols have metric structures and algorithms which are not compatible with other routing protocols. So in a large network which has many of the routers and routing protocols for communication, then it is very difficult to choose the best path for communication with another router because they have multiple protocols. Then at that time, Administrative distance helps a lot to choose the best path for communication. So here we learn about what is Administrative distance and how it works on routers.
What is the Administrative distance?
In simple and easiest language we can say that Administrative distance is an important feature that is used by routers to choose the best path for communication when two or more than two routers used two different routing protocols and routes to the same destination. Administrative distance also shows the reliability of a routing protocol which is used by routers to send a packet from source to destination. All the routing protocols like Static routing, RIP routing, EIGRP routing, OSPF routing and so on are prioritized in order of most to least reliable by using the Administrative value of the routing protocols.
How Administrative distance work on routers?
Administrative distance is the first and most important criteria which are used by routers to chose the best path for routers. It only occurs when two or more than two routing protocols are used for the route to the same destination. In which one is less reliable than other. But it is only defined only by the Administrative distance so routers use Administrative distance as first criteria to select the best path. The best thing about Administrative distance is it only has local significance and not advertised in routing updates. Administrative distance is a numeric value which can be ranging from 0 to 255. And this numeric value helps a lot to choose the reliable path for communication. If Administrative distance ha smaller value than it is more reliable than other routing protocols. So according to this the best reliable and trusted Administrative value is 0 and the worst Administrative value is 255.
For example, we take a network in which two routing protocols work on the routers and routes for the same destination. In which one is OSPF routing protocol (Open Shortest Path First routing protocol) which has an Administrative value that is “100” and other is Static routing protocol which has an Administrative value that is “1“. In that case, the router chooses Static routing protocol as the best path for sending packets from source to destination. Because Static routing protocol has less Administrative value than OSPF routing protocol. That means the router adds the Static version of the route to the routing table. If Static routing protocol fails to deliver information to the destination by some technical reasons then in that case router usage OSPF routing protocol to deliver information to the destination.
Administrative values can be set by default for all the routing protocols on all the routers. Here we can learn about all the Administrative values which are used by Cisco routers.
Default Distance Value Table for Cisco Routers
This table contains the default Administrative values for various routing protocols that Cisco supports:
|Routing Protocols||Administrative Distance|
|Directly connected interface||0|
|Static routing protocol||1|
|Dynamic Mobile Network Routing (DMNR)||3|
|Enhanced Interior Gateway Routing Protocol (EIGRP) summary route||5|
|External Border Gateway Protocol (BGP)||20|
|Open Shortest Path First (OSPF)||110|
|Intermediate System-to-Intermediate System (IS-IS)||115|
|Routing Information Protocol (RIP)||120|
|Exterior Gateway Protocol (EGP)||140|
|On Demand Routing (ODR)||160|
|Next Hop Resolution Protocol (NHRP)||250|
|Floating Static Route (ex. default backup route)||Variable–user assigned|
Note: – 1. If the Administrative value is 255, then the router does not believe on the source of that route and does not install the route in the routing table. It also removes the route from the routing table on the router and not use it.
2. Directly connected routes have an administrative distance of 0.
3. You can also modify the Administrative distance of a routing protocol on the router using the distance command in the routing process subconfiguration mode. We can learn about distance command in detail in upcoming articles.
4. Only the interface itself has an administrative value of 0 until a route cannot have a distance of less than 1.
These are some important facts about the Administrative distance of the routing protocol. In the next article, we learn about how to show and configure the Administrative value of a routing protocol. If you find this article helpful then share this article with your friends and if you have any queries regarding Administrative values than simply solved through the comment section. Also, follow our blog for further updates and stay updated.