Configuring EIGRP

Configuring EIGRP
The basic configuration of EIGRP is very similar to IGRP. To initialize EIGRP on the router,
you need to enter the following command in global configuration mode:
router eigrp AS#
AS# = any value between 1 and 65,535
Once you have initialized EIGRP on the router, you need to specify the interfaces you want to
include in the EIGRP AS process. In order to accomplish this, the following command needs to be
entered in router configuration mode:
network A.B.C.D
A.B.C.D = network number
Enhanced Interior Gateway Routing Protocol 129
In IOS version 12.0(4)T, Cisco started allowing the wildcard mask to be entered
along with the network address. A wildcard mask is an inverted subnet mask.
In other words, for every value of 1 in a subnet mask, the value for the same bit
in the wildcard mask would be 0.
This may get a little confusing since EIGRP is a classless routing protocol. In IOS versions
that do not allow you to enter the wildcard mask with the network command, the network
address entered must be of classful boundaries. This can cause a problem. When entering a
classful network address, you could possibly end up adding interfaces to EIGRP that you do not
want participating in EIGRP.
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 EIGRP AS. What happens is that the interface
on the WAN connection will send unneeded routing updates over the WAN connection. This
will not break anything, but it will use bandwidth.
The way to overcome this is to use the passive-interface command. This command
allows an interface’s subnet to be advertised in EIGRP updates but does not allow the interface
to send or receive them. You need to enter the following command under the routing protocol
process in router configuration mode to configure an interface as a passive-interface:
passive-interface interface
interface = the interface you don’t want to send or receive EIGRP updates.
One important concept you must be aware of is that EIGRP by default automatically summarizes
routes at the classful network boundary when the route crosses the classful network
boundaries. This enables EIGRP to easily work in conjunction with RIPv1 and IGRP. There are
times when you would prefer to have all subnets from classful networks advertised. The following
command needs to be entered in router configuration mode to disable automatic summarization
of routes:
no auto-summary
If you ever desire to turn automatic summarization back on, you just need to enter this command
without the no in front of it.
Now that we have an understanding of how to configure basic EIGRP, let’s take a look at
Figure 4.6.
For this exercise, you can assume that none of the routers are running a version of IOS that supports
the wildcard mask’s addition to the network statement. Also, the layer 2 technology has
already been configured. With that in mind, let’s walk through the configuration of these devices.
The configuration for the network would occur as follows:
Dallas>enable
Dallas#configure terminal
Dallas(config)#router eigrp 100
130 Chapter 4  IGRP and EIGRP
Dallas(config-router)#network 172.20.0.0
Dallas(config-router)#network 192.168.24.0
Richardson>enable
Richardson#configure terminal
Richardson(config)#router eigrp 100
Richardson(config-router)#network 192.168.24.0
Ft_Worth>enable
Ft_Worth#configure terminal
Ft_Worth(config)#router eigrp 100
Ft_Worth(config-router)#network 172.20.0.0
Plano>enable
Plano#configure terminal
Plano(config)#router eigrp 100
Plano(config-router)#network 172.20.0.0
FIGURE 4 . 6 Configuring basic EIGRP operation
EIGRP 100
S0
S1 S2
.2 S0
S0
.2 S0 .2
.1 .1
.1
192.168.24.0/24
172.20.192.0/24
172.20.128.0/24
Dallas
Richardson Ft_Worth Plano
Enhanced Interior Gateway Routing Protocol 131
Now that we have configured the network for EIGRP, let’s take a look at the routing table
to verify it has all the routes it needs:
Richardson#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 192.168.24.0/24 is directly connected, Serial0
D 172.20.0.0/16 [90/2297856] via 192.168.24.1, 00:00:02, Serial0
Richardson is displaying one route to network 172.20.0.0. The reason Richardson is displaying
one route for the network, and you know the network has been broken down into two subnets,
is because of default automatic summarization. In order for Richardson to see two routes
advertised, we need to disable automatic summarization on Dallas:
Dallas>enable
Dallas#configure terminal
Dallas(config)#router eigrp 100
Dallas(config-router)#no auto-summary
Now that we have disabled automatic summarization, let’s take a look at Richardson’s
routing table:
Richardson#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 192.168.24.0/24 is directly connected, Serial0
172.20.0.0/24 is subnetted, 2 subnets
D 172.20.128.0 [90/2297856] via 192.168.24.1, 00:00:02, Serial0
D 172.20.192.0 [90/2297856] via 192.168.24.1, 00:00:02, Serial0
132 Chapter 4  IGRP and EIGRP
There is one other way you can handle summarizing ranges in EIGRP. You can use the following
command, in interface configuration mode, to create summary addresses that are not at
the classful boundary. This is a very useful tool in large internetworks:
ip summary-address eigrp AS# address mask
AS# = the eigrp autonomous system number.
address = the summary aggregate address to apply to an interface.
mask = subnet mask
With this in mind, we will summarize 172.20.128.0/24 and 172.20.192.0/24 on Dallas into the
summary address of 172.20.128.0/17. We need to enter the summary command on interface S1
of Dallas:
Dallas>enable
Dallas#configure terminal
Dallas(config)#interface S1
Dallas(config-if)#ip summary-address eigrp 100 172.20.128.0 255.255.128.0
Now let’s take a look at the routing table on Richardson to verify the summary address:
Richardson#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 192.168.24.0/24 is directly connected, Serial0
172.20.0.0/17 is subnetted, 1 subnets
D 172.20.128.0 [90/2297856] via 192.168.24.1, 00:01:04, Serial0
As you can see, configuration of EIGRP is a straightforward process. Now that you know
how to configure EIGRP, we need to take a look at how to verify proper operation of EIGRP
and how to troubleshoot it if it isn’t operating properly.
Unlike IGRP, EIGRP supports update authentication. This topic is not covered in
this study guide because it is beyond the scope of the BSCI exam. However, it’s
something we believe you need to be aware of.