What you need to know about EIGRP
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.
Are You not very familiar with EIGRP? Let's look at some common things every network
administrator needs to know.
Know the EIGRP 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:
* show ip eigrp neighbors
* show ip eigrp interfaces
* show ip eigrp topology
* show ip eigrp traffic
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.