CCNA :: Part 5 - EIGRP Configuration

There are two modes from which EIGRP commands are entered: router configuration mode and interface configuration mode. Router configuration mode enables the protocol, determines which networks will run EIGRP, and sets global characteristics. Interface configuration mode allows customization of summaries, metrics, timers, and bandwidth. To start an EIGRP session on a router, use the router eigrp command followed by the autonomous system number of your network. You then enter the network numbers connected to the router using the network command followed by the network number.

Say you need to stop EIGRP from working on a specific interface, such as a BRI interface or a serial connection to the Internet. To do that, you would flag the interface as passive using the passive-interface interface command. Doing this will prohibit the interface from sending or receiving Hello packets and, as a result, stop it from forming adjacencies. This means that it won’t send or receive route information on this interface.

OK, let’s configure the known network that we configured in the last part with RIP. It doesn’t matter that RIPv2 are already running, because EIGRP has an AD of 90. But because of bandwidth consumption and CPU cycles I'll remove the RIP configuration.

The configuration for each router must be something like the following code:

R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router eigrp ?
 <1-65535> Autonomous system number

R1(config)#router eigrp 10
R1(config-router)#network 10.0.0.0
R1(config-router)#
R1(config-router)#end
R1#

The AS number, as you see can be any number from 1 to 65 535. A router can be a member of as many ASes as you want it to be.

In a second we will have the following routing table in R3

R3#sh ip route
Codes: C - connected, S - static, 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
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 4 subnets
D       10.10.1.0 [90/2684416] via 10.10.3.1, 00:05:23, Serial1/0
D       10.10.2.0 [90/2681856] via 10.10.3.1, 00:05:23, Serial1/0
C       10.10.3.0 is directly connected, Serial1/0
D       10.10.10.0 [90/2172416] via 10.10.3.1, 00:05:23, Serial1/0
R3#

Let's make the things a little bit dirty. I'll add a backup connection between R1 and R3, which will be connected through one of their FastEthernet interfaces. The picture will change like this:

The serial interfaces between R2, R and R3 are defined as 64 Kbps. EIGRP process knows this information and in the end we have the following routing table in R3:

R3#sh ip route
Codes: C - connected, S - static, 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
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 5 subnets
D       10.10.1.0 [90/30720] via 10.10.4.1, 00:06:50, FastEthernet0/1
D       10.10.2.0 [90/40517120] via 10.10.4.1, 00:06:50, FastEthernet0/1
C       10.10.3.0 is directly connected, Serial1/0
C       10.10.4.0 is directly connected, FastEthernet0/1
D       10.10.10.0 [90/2172416] via 10.10.3.1, 00:06:52, Serial1/0
R3#

Isn't it beautiful!? Now if Router R3 wants to go to network 10.10.1.0/24 he will go directly through router R1, because the connection is much faster and reliable. Let's see what will output the command sh ip eigrp topology:

R3#sh ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(10.10.4.3)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.10.1.0/24, 1 successors, FD is 30720
        via 10.10.4.1 (30720/28160), FastEthernet0/1
P 10.10.2.0/24, 1 successors, FD is 40517120
        via 10.10.4.1 (40517120/40514560), FastEthernet0/1
        via 10.10.3.1 (41024000/40512000), Serial1/0
P 10.10.3.0/24, 1 successors, FD is 2169856
        via Connected, Serial1/0
P 10.10.4.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 10.10.10.0/24, 1 successors, FD is 2172416
        via 10.10.3.1 (2172416/28160), Serial1/0
R3#

There are two routes for network 10.10.2.0/24, but that one through R1 is with beter Feasible Distance. So if something goes wrong with the direct connection between R1 and R3, R3 can still reach that netwoerk, but through the backup route, which is in the topology table. EIGRP can store up to six backup routes for each network. So advanced, right?