Enhanced Interior Gateway Routing Protocol - EIGRP

Introduction

Enhanced Interior Gateway Routing Protocol (EIGRP) is another Cisco proprietary, hybrid (has feature of Distance Vector and Link State protocols), interior gateway protocol (IGP) used by routers to exchange routing information. EIGRP uses a composite metric composed of Bandwidth, Delay, Reliability, and Loading to determine the best path between two locations.

EIGRP can route IP, IPX and Appletalk. Along with IS-IS, it is one of the few multi-protocol routing protocols.

The Diffusing Update Algorithm (DUAL) is the heart of EIGRP. In essence, DUAL always keeps a backup route in mind, in case the primary route goes down. DUAL also limits how many routers are affected when a change occurs to the network.

There is no maximum allowable number of hops. In a EIGRP network, each router multi-casts "hello" packs to discover its adjacent neighbor. This adjcency database is shared with other router to build a topology database. From the topology database the best route (Successor) and the second best route (Feasible Successor) is found.

EIGRP is classless, meaning it does include the subnet mask in routing updates. However, by default 'auto-summary' is enable. You must disable if you want subnet information from other major networks.

The EIGRP metric is a can be a complex calculation, but by default it only uses bandwidth and delay to determine the best path.