Network Administration / Networking

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.

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.

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