This article is the third in a series of articles covering the basics of Cisco networking. This article takes a look at the configuration of the Enhanced Interior Gateway Routing Protocol (EIGRP); EIGRP is a very popular routing protocol selection for those running only Cisco equipment on their routing networks.

EIGRP Concepts

EIGRP has been around for some time on Cisco equipment and for the most part is only supported on Cisco devices. This is an advantage and a disadvantage depending on what side of the debate you are on. EIGRP is an exceptional option as it mixes a number of the major advantages of other routing protocols including fast convergence, support for very large networks, efficient, support for load balancing (and unequal load balancing), as well as having a very simple configuration. The big problem however is that it is only supported on Cisco equipment. This is not a problem if your network utilizes only Cisco but if it utilizes any other vendor (or may in the future) it may be good to select one of the other options.

EIGRP is considered to be an advanced distance vector protocol (this used to be referred to as hybrid). This is because it features some traits from traditionally distance vector protocols and traditionally link state protocols. To start with, EIGRP uses the information gathered from its neighbors and bases its database on this information, it does not keep track of the whole network (like OSPF does). However, it does not broadcast out its known routes at specific intervals like both RIP and IGRP (EIGRP's predecessor). What EIGRP does is trade this routing information when forming a neighbor-ship with connected routers. If a new route becomes available then it is sent to all known neighbors (other than the one that it got the message from, if the route was remote) in a triggered update (which only contains the route itself not all other routes). What these differences offer is very fast convergence and minimal routing protocols traffic (it does send hello packets to track the state of neighbors).

EIGRP Configuration

One of the biggest advantages that EIGRP has over other highly scalable routing protocols is that its configuration is rather simple. The first step is to gain access to the Cisco device and access global configuration mode. If this process is not familiar, then read more about RIP configuration. Once this mode has been accessed the prompt should look like Figure 1.

Figure 1 - Global Configuration mode

To configure EIGRP, the first step is to create an EIGRP routing instance. This is done with the ‘router eigrp autonomous-system-number' command; this is shown in Figure 2.

Figure 2 - Creating a EIGRP router instance

It is important to note that the EIGRP autonomous system number is important because only neighbors with matching autonomous system numbers will communicate.

Like RIP, the next part requires knowledge of the networks that will be advertised from this device. To make it simple this article will use the same 192.168.1.0/24 network that is used in the RIP article. The command that is required for this is ‘network network'; this is shown in Figure 3.

Figure 3 - Configure the Advertised EIGRP network

If an interface exists using the IP network 192.168.1.0 with any mask then it will now be advertised with EIGRP. However, one other thing to note is that EIGRP has summarization capabilities including automatic summarization which is enabled by default. What this means is that if no other interface is configured on this device (the current device) using the 192.168.1.0 network then a summary route will be advertised in place of the more specific route. To make this clearer let's assume that a single interface on a router is configured with the IP address 192.168.1.129 and is using the /25 (255.255.255.128) mask. What this means is that the IP range from 192.168.1.128 through 192.168.1.255 is in the subnet connected off this interface. If no other interface is configured on this device than EIGRP will advertise the 192.168.1.0/24 network, not the 192.168.1.128/25 network. If this behavior is not wanted then EIGRP's automatic summarization must be disabled. The command to configure this is ‘no auto-summary' and is shown in Figure 4.

Figure 4 - Disabling EIGRP Automatic Summary

Summary

The configuration shown in this article is all that is required to set up EIGRP on a single device. Of course the IP address information may change and the number of interfaces is probably greater than one but the basic concepts are the same. Configuring EIGRP on basic networks is not a whole lot more complex than configuring a RIP network and a familiarity with EIGRP will be a much larger benefit to prospective future employers. Hopefully the content of this article will allow the reader to get started on their understanding of EIGRP and how it operates.