Appendix G. EIGRP (Enhanced Interior Gateway Routing Protocol)

The following topics are covered in this appendix:

• Neighbor Discovery and Recovery

• Reliable Transport Protocol

• DUAL Finite-State Machine

• Protocol-Specific Modules

• EIGRP Tables

• EIGRP Packet Types

• EIGRP Convergence

The Enhanced Interior Gateway Routing Protocol (EIGRP) is an evolution from its predecessor IGRP, resulting from the demands of diverse and large-scale internetworks. EIGRP integrates the capabilities of link-state and distance vector protocols.

EIGRP provides backward compatibility and seamless interoperation with IGRP routers. An automatic-redistribution mechanism allows IGRP routes to be imported into Enhanced IGRP and vice versa, making it possible to migrate EIGRP into an existing IGRP network. Because the metrics for both protocols are directly translatable, they are as easy to compare as though they were routes that originated in their own autonomous systems (ASes).

Key capabilities distinguishing Enhanced IGRP from other routing protocols include fast convergence, VLSM (variable-length subnet mask) support, partial route update support, and support for multiple network layer protocols, such as Novell's IPX or AppleTalk. Enhanced IGRP does not make periodic updates. Instead, it sends partial updates only when the metric for a route changes. Propagation of partial updates is automatically bounded so that only those routers that need the information are updated. As a result of these two capabilities, Enhanced IGRP consumes significantly less bandwidth than IGRP.

Enhanced IGRP includes support for AppleTalk, IP, and Novell NetWare. The AppleTalk implementation redistributes routes learned from Apple's Routing Table Maintenance Protocol (RTMP). The IP implementation redistributes routes learned from OSPF (Open Shortest Path First), RIP (Routing Information Protocol), IS-IS (Intermediate System-to-Intermediate System), or BGP (Border Gateway Protocol).

Cisco Systems, Inc. developed EIGRP, and like its predecessor IGRP (Interior Gateway Routing Protocol), EIGRP is Cisco proprietary. EIGRP is considered a hybrid routing protocol in that although it shares the distance-vector mechanisms found in IGRP, it differs greatly in the mechanics of operation. EIGRP introduces several features:

• Neighbor Discovery and Recovery

• Reliable Transport Protocol

• DUAL Finite-State Machine

• Protocol-Specific Modules

EIGRP also introduces a new route determination and update algorithm, the Diffusing Update Algorithm (DUAL). DUAL enables EIGRP routers to determine whether a path advertised by a neighbor is looped or loop free. DUAL also allows an EIGRP router to find alternative routes to destinations without waiting for routing updates from neighboring routers.

EIGRP, unlike IGRP, supports both VLSM (Variable Length Subnet Masking) and CIDR (Classless Inter-Domain Routing).

EIGRP was designed to be completely compatible with IGRP. EIGRP uses the same composite metrics as IGRP, as well as the same distance vectors and their respective mathematical weights. EIGRP and IGRP's metrics are directly comparable and can be used interchangeably after translation.

NOTE

The only difference between the IGRP and EIGRP algorithms is the calculation of the composite metric; IGRP is 20 bits long and EIGRP is 32 bits long, resulting in the EIGRP metric being 256 times larger than a comparable IGRP metric. The larger EIGRP metric enables a better and finer mathematical comparison of potential routes.