14: EIGRP Routing Protocol

EIGRP (Enhanced IGRP) is a hybrid routing protocol. It has some characteristics from both the distance vector routing and the link state routing.

EIGRP Characteristics

• It is a Cisco proprietary. It only works on Cisco devices.

• It is a classless routing protocol; this means that it supports the CIDR, the VLSM and the discontiguous networks.

• The maximum hop count is ‘255’.

• It uses ‘DUAL’ (Diffusion Update Algorithm) to find the best path to the networks.

• It can load balance between up to six unequal cost paths.

• It uses the ‘successor route’ and the ‘feasible successor route’ for fast convergence.

• The ‘successor route’ is the route that is installed in the routing table in order to use it to reach a certain network.

• The feasible successor route can be considered as a backup for the successor route.

EIGRP metrics

The default metrics used by the EIGRP protocol to calculate the path cost are the ‘delay’ and the ‘bandwidth’.

However, EIGRP protocol can use the delay, the bandwidth, the load, and the reliability as the path cost metrics.

EIGRP neighborship

• The routers that are running EIGRP discover neighbors using the ‘hello’ messages.

• The routers must have the following conditions to form a neighborship,

  1. 1. The same AS number (Autonomous System number); the autonomous system is the network that exists under the same administration.
  2. 2. The same path cost metrics.

• Only neighbors can exchange routes with each other using the multicast IP address ‘’.

EIGRP tables

Routers that are running EIGRP contain three tables,

The neighbor table: it contains the EIGRP neighbors.

The topology table: it contains the ‘EIGRP topology, including the successor and the feasible successor routes.

The routing table: it contains the routes that are currently used to route the data.

14.1 EIGRP configuration

To configure the EIGRP protocol on a router, we use the following commands,

Router(config)# router eigrp AS number

Router(config-router)# network IP address wildcard mask

Router(config-router)# no auto-summary

14.2 Administrative Distance (AD)

The ‘Administrative Distance’ (AD) is a number assigned to every routing protocol.

Figure (14.1), lists the ‘AD’ for every routing protocol,

Image2269.JPG Figure 14.1: the administrative distance

In figure (14.2),

Image2276.JPGFigure 14.2

Suppose that ‘R3’ has the RIP protocol and the OSPF protocol running on it.

The RIP protocol is telling R3 that, to reach the network ‘’, send the data to R1.

The OSPF protocol is telling R3 that, to reach network ‘’, send the data to R2.

Which path will R3 use to reach ‘’?

It will use the path that was installed in the routing table by the routing protocol that has the lowest AD.

Because the RIP protocol’s AD is ‘120’. Moreover, because the OSPF protocol’s AD is ‘110’, R3 will use the path installed by the OSPF protocol.

Therefore, to reach the network ‘’, R3 will send the data to R2.

14.3 Default route

Image2284.JPGFigure 14.3

In figure (14.3), suppose that R1 needs to send some data to a certain destination that exists in the internet. In addition, R1 searched for this destination in its routing table to determine the next hop. However, R1 did not find any entry for this destination in its routing table. What should R1 do?

In this case, R1 should send the data to the ‘default route’.

The default route is an entry in the routing table that is used by the router in case that it did not find any entry for the destination network in its routing table.

The default route entry in the routing table is ‘’ and the subnet mask is ‘’.

14.3.1 Default route configuration.

To configure the default route we use one of the following commands in the global configuration mode,

Router(config)# ip ute next hop IP address


Router(config)# ip route exit interface

Another method is to configure a default network is using the following command,

Router(confide)# ip default-network network IP address

In this method, the network IP must be reachable using one of the routing protocols.