6.1 EIGRP Concepts
6.1.2 EIGRP design
Cisco also instituted several changes in EIGRP that were designed to improve its operational efficiency relative to IGRP. These two protocols are interoperable thanks to their mutual distance vectors, but they operate in very different ways. EIGRP reacts to topological change differently, advertises routes differently, and even has a different approach to updating entries in routing tables. In many ways, EIGRP exhibits some behaviors of a link-state routing protocol. Yet, EIGRP uses the distance vectors and composite metric of IGRP. Consequently, EIGRP is sometimes referred to as a hybrid routing protocol (or an advanced distance-vector protocol). EIGRP combines the advantages of link-state and distance vector routing protocols. Properly designed and implemented, an EIGRP network is extremely stable and efficient and converges rapidly after any topological change.

As a hybrid protocol, some of the specific advantages of EIGRP include the following:

  • Rapid convergence --- EIGRP routers store every path they have learned to every destination in the network. Therefore, a router running EIGRP can quickly converge on an alternative route after any topological change. EIGRP uses an algorithm called the Diffusing Update Algorithm (DUAL) to achieve rapid convergence. A router running Enhanced IGRP stores backup routes, when available, for destinations so that it can quickly adapt to alternative routes. If no appropriate route or backup route exists in the local routing table, EIGRP queries its neighbors to discover an alternative route. These queries are propagated until an alternative route is found. The DUAL algorithm guarantees loop-free operation at every instant throughout a route computation and allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations. The convergence time with DUAL rivals that of any other existing routing protocol.
  • Efficient use of bandwidth during convergence --- EIGRP does not make periodic updates. Instead, it sends partial updates about a route when the path changes or when the metric for that route changes. When path information changes, the DUAL algorithm sends an update about that link only, rather than about the entire table. In addition, the information is sent only to the routers that need it, in contrast to link-state protocol operation, which sends a change update to all routers in an area. In EIGRP, this is known as a partial, bounded update.
  • Minimal consumption of bandwidth when the network is stable --- During normal, stable network operation, the only EIGRP packets exchanged between EIGRP nodes are hello packets. This simple handshake enables the EIGRP routers to know that all remains well in the network.
  • Support for VLSM and CIDR --- EIGRP supports the definition of network and host numbers on any bit boundary, per interface, for both IP addresses and subnet masks.
  • Complete independence from routed protocols --- EIGRP is designed to be completely independent of routed protocols. Support for routed protocols is via individual, protocol-specific modules. Therefore, evolution of a protocol, such as IP, won't threaten EIGRP with obsolescence. Nor will such technological advances force a painful revision of EIGRP.
  • Multiple network-layer support --- EIGRP supports AppleTalk, IP, and Novell NetWare through the use of protocol dependent modules (PDMs). These modules are responsible for network-layer-specific protocol requirements.