EIGRP Tutorial

EIGRP is a powerful routing protocol and it is really standout from its ancestor IGRP. The main features are listed below:

+ Support VLSM and discontiguous networks
+ Use Reliable Transport Protocol (RTP) to delivery and reception of EIGRP packets
+ Use the best path selection Diffusing Update Algorithm (DUAL), guaranteeing loop-free paths and backup paths throughout the routing domain
+ Discover neighboring devices using periodic Hello messages to discover and monitor connection status with its neighbors
+ Exchange the full routing table at startup and send partial* triggered updates thereafter (not full updates like distance-vector protocols) and the triggered updates are only sent to routers that need the information. This behavior is different from the link-state protocol in which an update will be sent to all the link-state routers within that area. For example, EIGRP will send updates when a new link comes up or a link becoming unavailable
+ Supports multiple protocols: EIGRP can exchange routes for IPv4, IPv6, AppleTalk and IPX/SPX networks
+ Load balancing: EIGRP supports unequal metric load balancing, which allows administrators to better distribute traffic flow in their networks.

* Notice: The term “partial” means that the update only includes information about the route changes.

EIGRP use metrics composed of bandwidth, delay, reliability, and load. By default, EIGRP uses only bandwidth and delay.

EIGRP use five types of packets to communicate:

+ Hello: used to identify neighbors. They are sent as periodic multicasts
+ Update: used to advertise routes, only sent as multicasts when something is changed
+ Ack: acknowledges receipt of an update. In fact, Ack is Hello packet without data. It is always unicast and uses UDP.
+ Query: used to find alternate paths when all paths to a destination have failed
+ Reply: is sent in response to query packets to instruct the originator not to recompute the route because feasible successors exist. Reply packets are always unicast to the originator of the query

EIGRP sends every Query and Reply message using RTP, so every message is acknowledged using an EIGRP ACK message.

Source

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