Cisco routing: Administrative distance(AD)

Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.

Select the Best Path

Administrative distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination. Administrative distance is a measure of the trustworthiness of the source of the routing information. Administrative distance has only local significance, and is not advertised in routing updates.

Note: The smaller the administrative distance value, the more reliable the protocol. For example, if a router receives a route to a certain network from both Open Shortest Path First (OSPF) (default administrative distance – 110) and Interior Gateway Routing Protocol (IGRP) (default administrative distance – 100), the router chooses IGRP because IGRP is more reliable. This means the router adds the IGRP version of the route to the routing table.

If you lose the source of the IGRP-derived information (for example, due to a power shutdown), the software uses the OSPF-derived information until the IGRP-derived information reappears.

Default Distance Values of different protocols are shown as below:

Connected interface: 0

Static route: 1

EIGRP summary route: 5

BGP: 20

Internal EIGRP: 90

IGRP: 100

OSPF: 110

IS-IS: 115

RIP:120

EGP: 140

On Demand Routing (ODR):160

External EIGRP: 170

Internal BGP:200

Unknown*:255

Note:If the administrative distance is 255, the router does not believe the source of that route and does not install the route in the routing table.

When you use route redistribution, occasionally you need to modify the administrative distance of a protocol so that it takes precedence. For example, if you want the router to select RIP-learned routes (default value 120) rather than IGRP-learned routes (default value 100) to the same destination, you must increase the administrative distance for IGRP to 120+, or decrease the administrative distance of RIP to a value less than 100.

You can modify the administrative distance of a protocol through the distance command in the routing process subconfiguration mode. This command specifies that the administrative distance is assigned to the routes learned from a particular routing protocol. You need to use this procedure generally when you migrate the network from one routing protocol to another, and the latter has a higher administrative distance. However, a change in the administrative distance can lead to routing loops and black holes. So, use caution if you change the administrative distance.

Exercises
There are three routes that can reach the same destination network. One is from EIGRP with a metric of 20514560. Another is from OSPF with a metric of 782. The last is from RIPv2 with a metric of 4. Which route will be installed by the router in its routing table?

A. the RIPv2 route
B. the EIGRP route
C. all three routes
D. the OSPF route
Answer: B

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