EIGRP Features, Operations, and Overview

This tutorial provides an overview of Enhanced Interior Gateway Routing Protocol (EIGRP) and compares it with the Routing Information Protocol (RIP) and the Interior Gateway Routing Protocol (IGRP). It examines the characteristics, features, functions, and fundamental operations of EIGRP.

Enhanced Interior Gateway Routing Protocol (EIGRP) is a routing protocol originally developed by Cisco as a proprietary solution. It operates on Cisco routers and certain Cisco multilayer switches. In 2013, Cisco released EIGRP as an open standard. EIGRP was designed to replace Interior Gateway Routing Protocol (IGRP) and to provide a more effective alternative to Routing Information Protocol (RIP).

Routing Information Protocol version 1 (RIP-1) was among the earliest and most widely adopted routing protocols, first published in 1988. RIP-1 broadcasts routing updates every 30 seconds from all enabled interfaces, with each update containing all routes from the routing table. Neighboring routers use these updates to maintain and modify their own routing tables.

The method by which a routing protocol determines the cost of a route is called the routing metric. Routing protocols may utilize either a single component or multiple components within their metric calculations.

RIP version 1 employs a single component in its routing metric: the hop count. The hop count represents the number of routers a packet traverses to reach its destination. For instance, if a packet passes through two routers, the hop count is 2.

When multiple routes to a destination exist, RIP selects the route with the fewest hops. RIP supports a maximum hop count of 15; routes exceeding this limit are considered unreachable.

RIP version 1 was primarily designed for small networks and is limited by its reliance on hop count as the sole metric. These constraints make it unsuitable for medium or large-scale networks. To address these limitations, Cisco developed IGRP as a more robust routing protocol.

IGRP incorporates multiple components into its routing metric, including bandwidth, delay, load, reliability, and maximum transmission unit (MTU), to determine optimal routes. It supports a maximum hop count of 255 (default: 100) and broadcasts routing updates every 90 seconds.

Although IGRP addressed several limitations of RIP, it was designed to meet the technological requirements of the 1980s. By the early 1990s, evolving business needs and advancements in networking technology necessitated further improvements. Consequently, Cisco introduced EIGRP in the mid-1990s as an enhanced successor to IGRP.

EIGRP is an updated version of IGRP and utilizes the same metric components: bandwidth, delay, load, reliability, and MTU. By default, only bandwidth and delay are active, while load, reliability, and MTU can be manually enabled. EIGRP incorporates all enabled components into its metric calculation algorithm.

While EIGRP and IGRP use the same metric components, EIGRP stores them in 32-bit format, whereas IGRP stores them in 24-bit format. Additionally, EIGRP introduces support for classless subnets, representing a significant enhancement.

IGRP operates as a classful routing protocol and does not include subnet information in its advertisements. In contrast, EIGRP is classless and incorporates subnet information in routing updates, enabling the advertisement of both classful and classless networks. This capability allows network administrators to implement Variable Length Subnet Masking (VLSM) within EIGRP networks.

Comparing RIPv1, IGRP, and EIGRP

The following table compares RIPv1, IGRP, and EIGRP.

Features of EIGRP

• It employs the concept of an autonomous system (AS), which refers to a group of contiguous routers operating the same routing protocol and exchanging routing information. By default, EIGRP does not share routing information with routers outside its autonomous system.
• It does not broadcast routing updates at regular intervals. Instead, it synchronizes network topology information with neighboring routers during initialization and subsequently transmits updates only when topology changes are detected.
• It supports a maximum hop count of 255, with a default value of 100. Unlike RIP, EIGRP does not use hop count as a routing metric; instead, hop count specifies the maximum number of routers a routing update can traverse before being discarded.

Characteristics of EIGRP

• It supports route summarization and VLSM/CIDR.
• It supports IPv4, IPv6, IPX, and AppleTalk routing.
• It provides a loop-free routing topology.
• It supports dis-contiguous networks.
• It does broadcast routing updates.

Functions and basic operations of EIGRP

• It uses the 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.

Conclusion

In summary, EIGRP offers substantial improvements over RIP and IGRP, including advanced metric calculation, support for classless routing, and efficient update mechanisms. These features enable scalable, reliable, and flexible routing, making EIGRP a preferred protocol for contemporary networks utilizing Cisco devices. A thorough understanding of EIGRP's features and enhancements enables more effective network design and management.