CCNA 46

EIGRP allows backup routes. It allows extremely fast convergence through the DUAL (Diffused Update Algorithm) algorithm. EIGRP keeps a topology table and remembers backup routes.
EIGRP is easy to configure and provides flexibility in summarization. Unlike OSPF, you can summarize anywhere. EIGRP allows unequal cost load-balancing through variance. EIGRP also supports multiple network protocols.

An EIGRP router maintains three tables:
-Neighbor Table
-Topology Table
-Routing Table

When the protocol is first run, EIGRP discovers neighbors. It is not as strict as OSPF so it can more easily form neighbors.

All the routes learned are put in the topology table. The best route to a location is marked as a successor (primary). The successor is put in the routing table. The next best route to a location is marked as a feasible successor (secondary). When the successor goes down, the feasible successor is immediately put into the routing table.

To configure EIGRP on a router, start by turning it on:
router eigrp 1

The number here must be the same throughout because it signifies the autonomous number. The autonomous number must be the same before the routers would form neighbor relationships.

There are two ways you can add networks into the process. You can either add it like RIP and type:
network 192.169.1.0

You can also use a wildcard mask with it:
network 192.169.0.0 0.0.255.255

The wildcard mask one is the one I'm going to use for R1. By default EIGRP does auto-summarization to the classful masks when it crosses different discontiguous networks (networks of different classes), so turn it off:
no auto

To see the neighbors of a router, use:
show ip eigrp nei

The output would be as follows:

IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.169.1.2             Fa0/1.1           14 00:03:44  107   642  0  18

The Hold column is the number of seconds EIGRP will keep a neighbor in the neighbor table without hearing a Hello. Hellos are sent every 5 seconds, so the Hold timer will count from 15 to 10 and back to 15 by default every 5 seconds. SRTT (Source Round Trip Time) is an estimate of the latency.

Remember to redistribute the static route:
redistribute static

Now to do manual summarization on R3, go into the S0/0 interface and type:
ip sum eigrp 1 172.16.0.0 255.255.248.0

The IP routing table on R1 would look like this now:

D       172.16.8.0/24
           [90/2300416] via 192.169.1.2, 00:16:49, FastEthernet0/1.1
D       172.16.0.0/21
           [90/2300416] via 192.169.1.2, 00:03:06, FastEthernet0/1.1

This is exactly what we want. Now go back to R3 and look at the routing table and you'll notice something quite peculiar:

C       172.16.8.0/24 is directly connected, Loopback8
C       172.16.4.0/24 is directly connected, Loopback4
C       172.16.5.0/24 is directly connected, Loopback5
C       172.16.6.0/24 is directly connected, Loopback6
C       172.16.7.0/24 is directly connected, Loopback7
C       172.16.0.0/24 is directly connected, Loopback0
D       172.16.0.0/21 is a summary, 00:03:37, Null0
C       172.16.1.0/24 is directly connected, Loopback1
C       172.16.2.0/24 is directly connected, Loopback2
C       172.16.3.0/24 is directly connected, Loopback3

Notice that there is a route to Null0. Null0 can be thought of as a garbage can. Any packet that is destined to 172.16.x.x but isn't as specific as currently available routes will be sent into Null0.