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IGRP Configuration
The basic configuration of IGRP is very straightforward
Nov 27,2008 00:00
by
alperen
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 |