Understanding Cisco IGRP and EIGRP Protocols

Understanding IGRP Protocol

What is IGRP?

IGRP is a Cisco-specific protocol developed in the mid-1980s.

IGRP Advantages Over RIP

IGRP calculates a composite metric from a set of route variables. It can be implemented on networks of up to 255 hops in diameter. Additional advantages include:

• Unequal-cost load sharing
• An update period three times longer than RIP's
• A more efficient update packet format

Unlike RIP, which uses UDP, the IGRP process is accessed directly from the IP layer as protocol 9.

IGRP Routing and Process Domains

Using domains allows isolating communications within one domain from others. IGRP uses two types: routing domains and process domains.

• Routing domains: Defined by Autonomous System (AS) boundaries and communicate via Exterior Gateway Protocols like BGP.
• Process domains: Defined by administrators within a routing domain, each identified by an IGRP process domain number.

Types of IGRP Routes

IGRP classifies route entries into three categories:

• Interior routes
• System routes
• Exterior routes

IGRP Composite Metric Explained

IGRP uses multiple metric parameters (a composite metric) based on link characteristics. These characteristics are:

• Bandwidth
• Delay
• Load
• Reliability

Exploring EIGRP Protocol

Key Components of EIGRP

EIGRP is a classless protocol with four key components:

• Protocol-Dependent Module
• Reliable Transport Protocol (RTP)
• Neighbor Discovery/Recovery
• Diffusing Update Algorithm (DUAL)

EIGRP Packet Reliability

EIGRP uses the Reliable Transport Protocol (RTP). Reliability is achieved through the Cisco-proprietary reliable multicast algorithm. Each neighbor receiving a reliably multicast packet unicasts an acknowledgment.

EIGRP vs RIP/IGRP Updates

Unlike RIP and IGRP updates, EIGRP update packets are transmitted only when necessary. They are unicast or multicast on demand and use reliable delivery.

EIGRP Neighbor Discovery

Hello messages are used to discover neighbors on directly connected networks. These messages are multicast every Hello interval (configured with ip hello-interval eigrp). Information about each neighbor is recorded in a neighbor table (viewed using the show ip eigrp neighbors command).

Related Networking Concepts

Aggregated Address Derivation

An aggregated address is derived by masking all the common bits of a group of numerically contiguous network addresses.

Understanding the Cisco Variance Command

The variance command specifies a route multiplier. A route metric can vary from the lowest-cost metric by this multiplier and still be included in an unequal-cost load balancing group.