When it comes to internal routing protocols, the two most
popular in use today at midsize and large companies are Enhanced Interior Gateway
Routing Protocol (EIGRP) and Open Shortest Path First (OSPF). Both of these
routing protocols offer a huge breadth of features that can cover just about
any routing scenario that a midsize or large company would require.

Not very familiar with EIGRP? Let’s look at some common
things every network administrator needs to know.

Know the basics

Let’s start with the fundamentals of EIGRP and discuss how
to configure this protocol.

What type of routing
protocol is EIGRP?
EIGRP is a hybrid-distance-vector routing protocol. It’s primarily a
distance-vector routing protocol, but it also uses the same composite metrics as
Interior Gateway Routing Protocol (IGRP). EIGRP uses the Diffusing-Update Algorithm
(DUAL) to perform look-free routing and calculate the shortest path.

How does EIGRP work?
With EIGRP, two routers form a neighbor relationship and exchange routes. Hello
packets (“keepalives”) are present between the two routers; they
serve to let each side know if the other goes down or if the link between them
goes down.

Typically, these keepalives between neighbors are multicast
packets. The type of multicast used is Reliable Transport Protocol (RTP), and
communication takes place using the reserved IP address 224.0.0.10.

How do I configure
EIGRP?
Like OSPF, EIGRP uses autonomous system numbers to identify areas of the network
that are under a single administrative domain. In other words, these network areas
are under the control of a single part of the company or a certain group.

To activate EIGRP on your router and enter its Configuration
Mode, use the router eigrp command
while in Global Configuration Mode. Here’s an example:

Router(config)# router eigrp {AS number}
Router(config-router)#

It doesn’t matter which Autonomous System (AS) number you
use—as long as it’s the same on all routers that will be talking to each other.
Valid options for the AS number are 1 to 65535. While you can configure more
than one AS on a single router, Cisco doesn’t recommend this approach

After entering the EIGRP Configuration Mode, a network
administrator’s most common task is to specify which networks EIGRP will
advertise. You can accomplish this using the network command. Here’s an example:

Router(config-router)# network 10.0.0.0 0.255.255.255

The first parameter is the network IP address; the second
parameter is the inverse mask. The inverse mask (or wildcard mask) is the
inverse of the subnet mask.

This command is similar to the OSPF network command. It tells OSPF which range of interfaces the specified
IP addresses will apply to. So, you can have one network statement that covers
multiple interfaces. However, unlike OSPF, EIGRP does not use areas.

How do I see what’s
going on with EIGRP?
After you’ve configured EIGRP, you need to know how to check its status. Here’s
a list of the most common EIGRP commands as well as links to their Cisco documentation:

Study the vocabulary

Now that you know how to configure EIGRP and you’re familiar
with common commands, let’s define some words you may run across while working
with EIGRP.

What is the topology
table?
Essentially, the topology table is the EIGRP database of available routes
received from neighbors. It shows the metric for these routes as well as the
feasible distance to these networks. The topology table contains a lot of
information about successors, feasible successors, and feasible distance.

What is a successor?
A successor is the neighbor with the best path to a destination.

What is a feasible
successor?
A feasible successor is the neighbor or neighbors that have other loop-free
paths to a destination that aren’t a preferred as the successor’s path.

What is the feasible
distance?
The feasible distance is the metric of a network advertised by the connected
neighbor plus the cost to get there.

What is an adjacency?
An adjacency is when two neighbors form a relationship and are exchanging
routes.

Get more specific

Now let’s take a look at some specifics about using EIGRP.

Does EIGRP use split horizon?
Split horizon is a loop-prevention method. Essentially, when using split
horizon, a routing protocol tries to prevent a routing loop. It does this by
not advertising a route from an interface from which it received an
advertisement for that route.

EIGRP uses split horizon, but you can disable it if
necessary. To do so, use the no ip
split-horizon eigrp {AS number} command. Keep in mind that the no ip split-horizoncommand doesn’t affect EIGRP, as it would RIP. Link-state routing
protocols such as OSPF and the Intermediate System-to-Intermediate System (IS-IS)
protocol don’t use split horizon.

Does EIGRP support
VLSM or CIDR?
EIGRP carries the subnet mask in the routing update, and it does support both variable
length subnet masks (VLSM) and Classless Inter-Domain Routing (CIDR). In other
words, you can subnet your network from the classful boundaries (where a class
A network is 10.0.0.0 with a 255.0.0.0 subnet mask, etc.), and EIGRP will work
fine. (Lack of support for VLSM and CIDR are limitations of RIP and IGRP.)

By default, EIGRP summarizes networks at the classful
boundaries. You can disable this by using the no auto-summary command in Router Configuration Mode.

What is the
administrative distance (AD) and routing table code for EIGRP?
An entry in the routing table for EIGRP looks something like the following:

D       10.93.103.0/24 [90/5542656] via 10.226.100.1, 00:30:54, Serial5/0

The D at the beginning
tells you that this is EIGRP. The 90 is
the
administrative distance for this EIGRP route. This is the default
administrative distance for EIGRP.

What happens when EIGRP
is “stuck in active”?
“Stuck in active” is a common issue with EIGRP. In fact, it’s so
common that Cisco has an acronym for it: SIA. Cisco has also created a support
page for SIA; however, Cisco login information is required.

Essentially, SIA occurs when an EIGRP router doesn’t receive
a query reply sent to its neighbor after three minutes. When this happens, you’ll
see “DUAL-3-SIA” in the log file. Troubleshooting this can be quite complex,
so I would refer to the Cisco documentations.

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David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.