If you are just looking into routing protocols for use in your network, one of the options that should be at the top of the list is EIGRP. The beauty of EIGRP is that it can be used in a small network all the way up to a multi-site large enterprise network.

Configuring this protocol can be the same story, the basic setup needed is very simple to configure - but has the options for configuring the advanced features used in larger networks.

First let’s start with a basic network containing three routers. The Lan and Wan Routers are in the same ‘Main’ office with the BranchRouter located at a secondary location. Even though this network could be routed easily with static routes, we want to go ahead and start with a routing protocol to be prepared for future expansions..

hostname LanRouter
!
interface FastEthernet0/0
description Infrastructure Subnet
ip address 10.0.1.1 255.255.255.0
!
interface FastEthernet1/0
description Employee Subnet
ip address 10.0.2.1 255.255.255.0

hostname WanRouter
!
interface FastEthernet0/0
description Infrastructure Subnet
ip address 10.0.1.2 255.255.255.0
!
interface Serial1/0
description Point to Point link to Branch Office
ip address 192.168.1.1 255.255.255.252

hostname BranchRouter
!
interface FastEthernet0/0
description Branch Employee Subnet
ip address 10.1.2.1 255.255.255.0
!
interface Serial1/0
description Point to Point link to Main Office
ip address 192.168.1.2 255.255.255.252

If we want to route between these two offices, here is all the routing configuration that is needed:

LanRouter(Config)#router eigrp 1
LanRouter(Config)#network 10.0.1.0 0.0.0.255
LanRouter(Config)#network 10.0.2.0 0.0.0.255
LanRouter(Config)#no auto-summary

WanRouter(Config)#router eigrp 1
WanRouter(Config)#network 10.0.1.0 0.0.0.255
WanRouter(Config)#network 192.168.1.0 0.0.0.3
WanRouter(Config)#no auto-summary

BranchRouter(Config)#router eigrp 1
BranchRouter(Config)#network 10.1.2.0 0.0.0.255
BranchRouter(Config)#network 192.168.1.0 0.0.0.3
BranchRouter(Config)#no auto-summary

For the EIGRP configuration, the network statements encompass the interface IP addresses, which tells the router you want to run EIGRP on any connected interface that falls within that network statement.  It is considered best practice to specify host network statements, instead of an entire classless/classfull statements (as was done above). The reason not to do this is because a new interface could be created inside a specified classless network statement - which would enable the routing protocol for that interface. The result may or may not be desired, so the alternative is to specifically specify the interface IP’s.  By doing this, you know exactly which interfaces belong to a particular routing protocol instance.

To enable just the FastEthernet0/0 interface in the EIGRP routing process, the appropriate network statement would look like:

LanRouter(Config)#router eigrp 1
LanRouter(Config)#network 10.0.1.1 0.0.0.0

The no auto summary command used above is needed to prevent EIGRP from automatically summarizing the routing advertisements at the classful boundary. For example, without the no auto-summary command, the LanRouter would send the 10.0.1.0 network advertisement as 10.0.0.0/8, instead of 10.0.1.0/24.

The configuration explained in this article can be simulated/tested by using the following Dynamips lab:

Dynagen Configuration using Three 1721 Routers