EIGRP is a Cisco proprietary protocol that combines the advantages of link-state and distance vector routing protocols. EIGRP has its roots as a distance vector routing protocol and is predictable in its behavior. Like its predecessor, Interior Gateway Routing Protocol ( IGRP ), EIGRP is easy to configure and is adaptable to a wide variety of network topologies. The addition of several link-state features, such as dynamic neighbor discovery, makes EIGRP an advanced distance vector protocol. While EIGRP is compatible with existing IGRP networks, EIGRP is an enhanced IGRP because of its rapid convergence and the guarantee of a loop-free topology at all times. A hybrid protocol, EIGRP uses the Diffusing Update Algorithm (DUAL) and includes the following key features:
Fast convergence: A router running EIGRP stores all its neighbors’ routing tables so that it can quickly adapt to alternate routes. If no appropriate route exists, EIGRP queries its neighbors to discover an alternate route. These queries propagate until an alternate route is found.
VLSM support: EIGRP is a classless routing protocol, which means that it advertises a subnet mask for each destination network; this structure enables EIGRP to support discontinuous subnetworks and VLSM. With EIGRP, routes are automatically summarized at the major network number boundary, but EIGRP can be configured to summarize on any bit boundary on any router interface.
Partial updates: EIGRP does not send periodic updates. Instead, it sends partial triggered updates; these are sent only when the path or the metric changes for a route, and they contain information about the changed routes only. 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, EIGRP consumes significantly less bandwidth than IGRP. This behavior is different from that of link-state protocols, in which an update is transmitted to all link-state routers within an area.
Multiple network-layer protocol support: EIGRP supports IP, AppleTalk, and Novell NetWare Internet Packet Exchange (IPX) through the use of protocol-dependent modules. These modules are responsible for protocol requirements specific to the network layer. The rapid convergence and sophisticated metric structure of EIGRP offers superior performance and stability when implemented in IPX and AppleTalk networks.
Multicast and unicast: EIGRP uses multicast and unicast, rather than broadcast. The multicast address used for EIGRP is 224.0.0.10.