What Is EIGRP Administrative Distance (AD)?

Introduction

Administrative distance is the feature used by routers 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) using an administrative distance value.

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 Enhanced Interior Gateway Routing Protocol (EIGRP) (default administrative distance – 90), the router chooses EIGRP because EIGRP is more reliable.

Note – We can notice that EIGRP is using three AD values which are, Internal-EIGRP 90, External-EIGRP 170, and AD value for EIGRP summary route is 5.

The administrative distance can be modified. When you use route redistribution, occasionally you need to modify the administrative distance of a protocol so that it takes precedence. You can modify the administrative distance of a protocol through the distance command in the routing process sub configuration mode. This command specifies that the administrative distance is assigned to the routes learned from a particular routing protocol.

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.

EIGRP LAB Topology

Topology is having four devices running with EIGRP AS number 1 as a dynamic routing protocol. The configuration tasks are as such that all of EIGRP AD values which are 5, 90, and 170 may be verified.

Cloud devices is advertising routes 4.4.4.1/32 to 4.4.4.7/32 through network command and R1 is advertising the 100.1.1.1/32 to 100.1.1.4/32 through redistribution. In turns On R2 we are summarizing the routes received from cloud while sending towards R3.

Task1: Default AD value

When advertising the routes through EIGRP network command, we will see this output in-between cloud and R1 device where cloud is publishing the 4.4.4.x networks.

Cloud configuration on loopback 0

We are configuring the EIGRP named mode for the cloud with ASN 1.

If we check on R1 about any of the cloud route information we can see these are internal routes and having AD value as 90. This is considered as default value when we are not redistributing or summarizing.

Task2: Redistribution

On R1 there are four prefixes available as loopback which are 100.1.1.1/32, 100.1.1.2/32, 100.1.1.3/32, 100.1.1.4/32. Need to write an ACL to filter specific routes and can match in the route-map so that may use during our redistribution command.

Standard ACL to filter prefixes

Route-map to match ACL

Performing redistribution

Output on router R2 for any of 100.1.1.x prefixes which is advertised through redistribution will show that it is external EIGRP route and has the AD value as 170.

Same output is visible in routing table as well, where eigrp external routes have default AD of 170.

Task3: Distance Manipulation (Traffic Engineering)

In some scenarios it may have the requirement to change the default AD values, like in this task we are changing AD value from 170 to 111 for the external routes

EIGRP configuration is simple, where for internal routes we are not changing and putting default as 90 but for external it is changed to 111.

Output in routing table at once reflect the new changes for AD value for the external routes

Task4: Summarization

Let’s summarize all the cloud routes while sending it to the R3 on R2, and we will check the AD for summary route would be 5.

Currently on R2 all the routes from cloud is received as normal eigrp routes and so with default internal AD as 90.

While if we need to send summary only towards R3, we need to add the summary command on the interface connected with the device R3.

When summary route is added for any protocol, the null entry gets created automatically which we can observe here as well.

While verifying the AD value for this summary route it is clear by default the summary distance would be 5.

Please be noted: – While checking the routing table now on R3 we wouldn’t have specific routes but only summary is there and the AD value of summary on destined device is 90 which is considered as internal only.

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