IGRP Configuration The basic configuration of IGRP is very straightforward

IGRP Configuration
The basic configuration of IGRP is very straightforward. In order to initialize IGRP on a router,
enter the router igrp AS# command in global configuration mode:
Router(config)#router igrp ?
<1-65535> Autonomous system number
Once you’ve initialized IGRP on the router, you need to specify the interfaces that you want
to include in the IGRP AS process, through the use of the classful network. In order to accomplish
this, enter the network A.B.C.D command in router configuration mode:
Router(config-router)#network ?
A.B.C.D Network number
IGRP expects classful network addresses, so entering subnet addresses has no
additional effect. In fact, it would be considered incorrect to do so. IGRP watches
out for you, however, by converting the classless entry to a classful one, as can
be seen with the show run or show ip protocols commands.
IGRP 100 EIGRP 100
Ethernet
Router1
Router2
Ethernet
Router3
192.168.1.0/24 192.168.200.0/24
192.168.24.0/24 192.168.150.0/24

That’s all that is needed to configure basic IGRP.
You can also configure IGRP so it will send unicast updates to its neighbors. A unicast update
is sent directly to the specified neighbor instead of being broadcast, cutting down on the amount
of bandwidth utilized for updates. In order to configure unicast updates, enter the neighbor
A.B.C.D command in router configuration mode:
Router(config-router)#neighbor ?
A.B.C.D Neighbor address
As noted earlier, IGRP’s network command pays attention only to the classful portion of the
address you enter. So when you use the network statement, any interface on the router that falls
into the classful network range will be included in the IGRP routing domain. This causes a problem
when you don’t want one of the interfaces in the classful range to participate in the IGRP AS.
This type of scenario could arise when you are running another routing protocol over one of your
WAN connections and the interface participating in that connection is in the same classful network
range as an interface participating in the IGRP AS.
Without the neighbor command to force unicasts to the WAN neighbor, the interface will
broadcast updates over the WAN link. The router on the other end of the link receives the
update packet and drops it, because that router’s interface isn’t participating in the IGRP AS.
This will not break anything, but it wastes bandwidth.
The way to overcome this is to use the passive-interface command. The passiveinterface
command allows an interface’s associated network—and possibly subnet—to
be advertised in the IGRP routing domain, but the interface will not listen to or send IGRP
updates itself. You need to enter the passive-interface interface_type interface_
number command under the IGRP process in router configuration mode to configure an
interface as a passive-interface (interface Serial 0/0 shown):
Router(config-router)#passive-interface s0/0
Now that you have a basic understanding of how to configure IGRP, let’s walk through an
actual configuration. Take a look at Figure 4.3.
Let’s assume for the given network that the layer 3 interface addresses are already configured.
We’re going to concentrate only on the routing protocol for this example.
The configuration for the network would occur as follows:
Router1>enable
Router1#configure terminal
Router1(config)#router igrp 100
Router1(config-router)#network 172.16.0.0
Router1(config-router)#network 192.168.100.0
Router2>enable
Router2#configure terminal
Router2(config)#router igrp 100
Router2(config-router)#network 192.168.100.0
Interior Gateway Routing Protocol 113
Router2(config-router)#network 192.168.21.0
Router3>enable
Router3#configure terminal
Router3(config)#router igrp 100
Router3(config-router)#network 172.16.0.0
Router3(config-router)#network 192.168.21.0