EIGRP – Distance Vector Routing Protocol

EIGRP was developed from IGRP, another distance vector protocol. EIGRP is a classless, distance vector routing protocol with features found in link-state routing protocols. However, unlike RIP or OSPF, EIGRP is a proprietary protocol developed by Cisco and runs only on Cisco routers.

EIGRP features include

■ Triggered updates (EIGRP has no periodic updates).

■ Use of a topology table to maintain all the routes received from neighbours (not only the best paths).

■ Establishment of adjacencies with neighbouring routers using the EIGRP Hello protocol.

■ Support for VLSM and manual route summarisation. These allow EIGRP to create hierarchically structured large networks.

Advantages of EIGRP are as follows:

■ It can perform unequal-cost load balancing.

■ Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path, rather than hop count.

Fast convergence because of Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers.

■ Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes.

Configuring EIGRP
To configure EIGRP, the first step is to create an EIGRP routing instance. This is done with the ‘router eigrp autonomous-system-number’ command

It is important to note that the EIGRP autonomous system number is important because only neighbours with matching autonomous system numbers will communicate. You are free to use any number between 1 and 65535

Like RIP, the next part requires knowledge of the networks that will be advertised from this device. To make it simple we will use the same 10.0.0.0/8 and 192.168.1.0/24 network that is used in the RIP above. The command that is required for this is again ‘network’;

If an interface exists using the IP network 192.168.1.0 with any mask then it will now be advertised with EIGRP. However, one other thing to note is that EIGRP has summarisation capabilities including automatic summarisation which is enabled by default. What this means is that if no other interface is configured on this device (the current device) using the 192.168.1.0 network then a summary route will be advertised in place of the more specific route. To make this clearer let’s assume that a single interface on a router is configured with the IP address 192.168.1.129 and is using the /25 (255.255.255.128) mask. What this means is that the IP range from 192.168.1.128 through 192.168.1.255 is in the subnet connected off this interface. If no other interface is configured on this device than EIGRP will advertise the 192.168.1.0/24 network, not the 192.168.1.128/25 network. If this behaviour is not wanted then EIGRP’s automatic summarisation must be disabled. The command to configure this is ‘no auto-summary’

Example of use

Lets say you wanted to configure your router to use EIGRP to advertise two directly connected networks of 10.0.0.0 and 192.168.0.0 the following commands would be used.

Router(config)#router eigrp 100
Router(config-router)network 10.0.0.0
Router(config-router)network 192.168.1.0

Next – Link-State Routing Protocol