Layer 3 technologies - Describe administrative distance

Exam: Cisco 300-101 - CCNP Implementing Cisco IP Routing (ROUTE v2.0)

In this chapter we will discuss about how to describe the administrative distance. This is an important topic that you will come across in case you are preparing for the CCNP exam. This comes under the main topic of “layer 3 technologies”. The topic is important from the 300-101 Route exam point of view. We hope that you will find the explanation in this chapter adequate.

You will often notice that most of the routing protocols will have metric structures that are generally not compatible with other protocols. If a network has multiple routing protocols then the capability to select the best path and exchanging of the route information are both very critical.

The administrative distance is nothing but a feature that the routers will use in order to choose the best path. This works best when there is more than one different route that leads to the same destination. With the help of the administrative distance you can also understand the reliability of the routing protocol. All the routing protocols are prioritized on the basis of the most to the least reliable this can be done with the help of administrative distance value.  The administrative distance is also reffered to as AD.

You can also say that the administrative distance is nothing but a measure of how trust worthy the source of the routing information actually is. The administrative distance only has a local significance that is why it is not advertised in the routing updates. You must make a note here that the smaller the administrative distance is the more reliable the protocol will be. The router will mostly add the IGRP version.

IGRP means Interior gateway routing protocol. It may happen that you lose the very source of the IGRP information. This can happen if there is a sudden power shutdown. In this case the software will have to use the OSPF derived information till the IGRP derived information is gathered again. The routes that are issued by EIGRP are generally considered to be more reliable than the ones that are issued by the RIP. This is because the EIGRP will have an administrative distance of 90 and the RIP has an administrative distance of 120.

As a network administrator you will have the authority to manually set the administrative distance. This is a very crucial task for the network administrator. If the network administrator fails to set the administrative distance properly then the router will be forced to use the default administrative distance.

If the administrative distances are not proper they can lead to results that are absolutely not expected. It can also select a route that is not necessarily the best route available. This happens more often if the routing distribution is used on the router. If this happens the network administrator will have to manually modify the administrative distance this will help the desired routing protocols to take precedence over the other routes.

You can also refer to the default distance value table. It gives the list of the administrative distance default values of the protocols that the Cisco supports. This table can be very useful for the network administrator. The table gives the routing protocol and the respective administrative distance.

If the administrative distance is 255 it will make the router to disbelieve the route completely as a result it will not use it at all. The router will remove the route with administrative distance of 255 from the RIB.

The static route will always have an administrative distance of 1 if it has an exit interface. The interface itself is the only one that will have an administrative distance of 0. This is mainly because the route cannot be less than 1. Here you must also know that the directly connected routes will always have an administrative distance of 1.

The directly connected routes will always have an administrative distance of 1 this can also be proved by using the show ip route command in a privileged exec mode.

 If you are using the route redistribution then you will sometime need to use modify the administrative distance of the protocol this will help it to take the precedence. The administrative distance of a protocol can be modified by using the distance command.

This must be done to the routing process during the subconfiguration mode itself. This command will also help you to specify that the administrative distance is assigned to the routes that are learned from the particular routing protocol. This is a process that must be followed whenever you are shifting from one routing protocol to the other. It is more effective when the later protocol has a higher administrative distance. One must know here that a change in the administrative distance can lead to black holes and routing loops. This is a caution that must be followed when you are changing the administrative distance.

As all the networks have their own special requirements there is no rule that you can follow while assigning the administrative distance. This is exactly why you must choose a reasonable matrix for the administrative distances of the whole network.

One common reason that is listed for changing the administrative distance of the route is when there is a need to use static routes to backup the existing IGP route. This is mostly done in order to bring up a backup link when the primary link fails.

These are more or less all that you need to know about the topic of describing administrative distance. We hope that you found this chapter informative and interesting. We wish you all the best for the exam hope you will do well.

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