This tutorial introduces EIGRP and compares it with RIP and IGRP. Learn the characteristics, features, functions, and basic operations of EIGRP.
EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco proprietary routing protocol. It runs only on Cisco routers and on some Cisco multilayer switches. In 2013, Cisco converted the functionality of EIGRP to an open standard. Cisco developed EIGRP to update IGRP (Interior Gateway Routing Protocol) and IGRP to provide a better option than RIP.
Historically, RIPv1 was the first most popular routing protocol. It was published in 1988. It broadcasts a routing update every 30 seconds from all enabled interfaces. A routing update includes all routes from the routing table. A neighboring router uses routing updates to update its routing table.
The formula a routing protocol uses to calculate the cost of a route is called the routing metric. A routing protocol can use a single component or multiple components in the routing metric.
RIPv1 uses only one component in the routing metric. It counts the number of routers in the route to calculate the best route to reach each destination. The number of routers in a route is known as hop count. A hop is a default gateway in the path that the packet cross to reach the destination. For example, if there are two routers in the path, the hop count will be 2.
If there are two routes to a destination, it chooses the route that has less hop count. It can count a maximum of 15 routers. It means if a route has more than 15 routers, it will not use the route.
RIPv1 was mainly developed for small networks. It has many limitations and uses only hop count in the metric. Because of these, RIPv1 was not a good choice for mid or large-size networks. Cisco developed IGRP to overcome the limitations of RIPv1 and provide a better routing protocol.
IGRP uses multiple components in the routing metric to select the best route for each destination. These components include bandwidth, delay, load, reliability, and MTU. IGRP supports a maximum 255 hop count (default 100). It broadcast routing updates every 90 seconds.
Like RIPv1, IGRP also had some technical limitations. It was developed to support the technology levels of the 1980s. Till the 1980s, it was a good option. In the early 1990s, business requirements and technical factors pushed Cisco to update IGRP. In the mid-1990s, Cisco updated IGRP to EIGRP.
EIGRP is the updated version of IGRP. EIGRP uses the same metric components. These components are bandwidth, delay, load, reliability, and MTU. By default, only bandwidth and delay are enabled. However, you can manually enable load, reliability, and MTU. EIGRP uses all enabled components in the metric algorithm.
Even though EIGRP uses the same metric components, it stores them in different size values. IGRP uses a 24-bit metric, while EIGRP uses a 32-bit metric. Another major enhancement in EIGRP is that EIGRP supports classless subnets.
IGRP is a classful routing protocol. It does not advertise subnet information. EIGRP is a classless routing protocol. It includes subnet information in routing updates. It can advertise both classful and classless networks. This feature allows network administrators to use VLSM in EIGRP networks.
EIGRP uses the concept of an AS (autonomous system). An AS describes a set of contiguous routers that run the same routing protocol and share routing information. By default, EIGRP does not share routing information with different AS routers.
EIGRP does not advertise routing updates periodically. it synchronizes network topology information between neighbors at start-up and then sends specific updates only when topology changes occur.
EIGRP also supports a maximum 255 hop count (default 100). Unlike RIP, EIGRP does not use hop count as a metric. EIGRP uses hop count to refer to how many routers an EIGRP routing update can go through before it will be discarded.
Comparing RIPv1, IGRP and EIGRP
The following table compares RIPv1, IGRP, and EIGRP.
| Option | RIPv1 | IGRP | EIGRP |
| Type of protocol | Open-source | Cisco proprietary | Cisco proprietary |
| Metric | Hop count | Bandwidth, delay, reliability, load, MTU | Bandwidth, delay, reliability, load, MTU |
| Bits to store metric value | - | 24bits | 32bits |
| The maximum supported router in a route | 15 | - | - |
| Maximum number of routers a routing update can go | - | 255 | 255 |
| Supported network size | small | all | all |
| Routing update method | Broadcast | Multicast | Multicast |
Characteristics and features
- It supports route summarization and VLSM/CIDR.
- It supports routing for IPv4, IPv6, IPX, and AppleTalk.
- It provides a loop-free routing topology.
- It supports dis-contiguous networks.
- It does broadcast routing updates.
Functions and basic operations
- It uses Diffusing Update Algorithm (DUAL) to build a loop-free routing topology.
- It uses multicast to advertise routing updates.
- It can load balance across six paths to a destination.
- It uses triggered updates when changes occur.
That’s all for this tutorial. In this tutorial, we compared EIGRP with RIPv1 and IGRP and learned some basic characteristics and features of EIGRP.
