Introduction to Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary enhanced Distance Vector routing protocol. EIGRP is based on IGRP, hence the configuration is similar. Enhanced Interior Gateway Routing Protocol (EIGRP) is considered as a Hybrid Routing Protocol because EIGRP has characteristics of both Distance Vector and Link State Routing Protocols. Both EIGRP and IGRP offer load balancing across six paths (equal or unequal), and they have similar metric structures. EIGRP has faster convergence, and has less network overhead, since it uses incremental updates. Another important features of Enhanced Interior Gateway Routing Protocol (EIGRP) are routing loop-free topology, VLSM and route summarization, multicast and incremental updates and routes for multiple routed protocols (IP, IPX and AppleTalk)

Enhanced Interior Gateway Routing Protocol (EIGRP) Uses Diffused Update Algorithm (DUAL) to calculate the shortest path.

The following formula is used to calculate the metric of Enhanced Interior Gateway Routing Protocol (EIGRP).

Metric = [K1*Bandwidth + (K2*Bandwidth)/ (256 - Load) + K3*Delay] * [K5/(Reliability + K4)]

The default values for K are K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0. For default behaviour, the formula can be simplified as metric = bandwidth + delay

Important terms related with Enhanced Interior Gateway Routing Protocol (EIGRP)

DUAL

DUAL stands for Diffused Update Algorithm, the algorithm used by Enhanced Interior Gateway Routing Protocol (EIGRP) to calculate the shortest path.

Neighbor table

Neighbor table contains a list of the EIGRP neighbours. Each routed protocol for EIGRP has its own neighbour table.

Topology table

Topology table contains a list of all destinations and paths the EIGRP router learned. There is a separate topology table for each routed protocol.

Successor

Successor is the best path to reach a destination within the topology table.

Feasible successor

Feasible successor is the best backup path to reach a destination

Routing table

Routing table contains all of the successor routes from the topology table. There is a separate routing table for each routed protocol.

Advertised distance

Advertised distance is the distance (metric) that a neighbouring router is advertising for a specific route.

Feasible distance

Feasible distance is the distance (metric) that your router will use to reach a specific route.