Introduction to EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) was introduced as a distance-vector routing protocol in 1992. It was originally designed to work as a Cisco proprietary protocol on Cisco devices only. In 2013, Enhanced Interior Gateway Routing Protocol became a multi-vendor routing protocol.

Enhanced Interior Gateway Routing Protocol is an advanced version of IGRP that lets routers exchange information more efficiently than then previous network protocols.  As the name suggested, Enhanced Interior Gateway Routing Protocol is an enhancement of IGRP(Interior Gateway Routing Protocol). IGRP is obsolete since IOS 12.3 release. It was a classful, distance vector routing protocol.

Enhanced Interior Gateway Routing Protocol is a distance vector routing protocol with features of link-state routing protocols. It is suitable for many different topologies and media. In a well-designed network, Enhanced Interior Gateway Routing Protocol can scale to include multiple topologies and can provide extremely quick convergence times with minimal network traffic.

Enhanced Interior Gateway Routing Protocol is also known as the hybrid routing protocol in some older documentation. But this term is false because Enhanced Interior Gateway Routing Protocol is not a hybrid between distance vector and link-state routing protocols. Enhanced Interior Gateway Routing Protocol is only a distance vector routing protocol.

Routers using either Enhanced Interior Gateway Routing Protocol and IGRP can interoperate because the metric used with one protocol can be translated into the metrics of the other protocol. Enhanced Interior Gateway Routing Protocol can be used not only for Internet Protocol (IP) networks but also for AppleTalk and Novell NetWare networks.

Features of EIGRP

Enhanced Interior Gateway Routing Protocol is a routing protocol which includes features of both link-state and distance vector routing protocols. However, the key principles of Enhanced Interior Gateway Routing Protocol are still based on the key distance vector routing protocol, because it gets information from directly connected neighbours. EIGRP is an advanced version of the distance vector routing protocol because it includes features not found in any other distance vector routing protocols.

Diffusing Update Algorithm (DUAL)

The diffusing update algorithm (DUAL) is the algorithm used by EIGRP routing protocol to make sure that a given route is recalculated globally whenever it might cause a routing loop. It is guarantees loop-free and backup paths throughout the routing domain. EIGRP store all available backup routes using DUAL and then adapt the route when needed.

Establishing Neighbor Adjacencies

EIGRP establishes Neighbor adjacencies with directly connected routers that are also enabled EIGRP to track the status of these directly connected routers.

Reliable Transport Protocol

EIGRP depends upon proprietary protocol Reliable Transport Protocol (RTP) to provide delivery of EIGRP packets to neighbours. RTP and the tracking of neighbour adjacencies set the stage for DUAL.

Partial and Bounded Updates

Unlike RIP, EIGRP not sending any periodic updates and route entries because it does not age out. It uses “partial and bounded” terms for updates. The term partial use for update only includes information about the route changes, for example, a new link or a link becoming unavailable. The term bounded means propagation of partial updates that are sent only to those routers that the changes affect. Partial and bonded update minimizes the bandwidth for EIGRP updates.

Equal and Unequal Cost Load Balancing

EIGRP supports both equal and unequal cost load balancing. Equal and Unequal cost load balancing allows better traffic flow in the network. It is possible due to changing the value of variance. The default, the variance is 1, therefore, supports equal-cost load balancing but if we want to use unequal cost load balancing then we can change the value of variance according to the amount of traffic we want to split across different paths.