EIGRP protocol

1. Overview:
   Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced version of IGRP developed by Cisco; it provides superior convergence properties and operating effiency, and combines the advantages oflink-state protocols with those of distance-vector protocols.
2. Purpose of EIGRP:
   
   Early routing protocols were based on distance vectors; they were very simple and easy to implement but had the severe drawbacks of counting to infinity and routing loops. These problems were reduced using techniques such as split horizon, holddowns, and so on. These techniques, however, introduced long convergence times. Routing protocols based on link states have been implemented to address the problem of slow convergence in distance-vector protocols, but they add complexity in configuration and troubleshooting. EIGRP is an advanced distance-vector protocol that scales well, is easy to configure, and provides extremely quick convergence times with minimal network traffic.
   EIGRP is a classless protocol, meaning that it supports variable-length subnet mask (VLSM) and aggregation. An aggregate is a summarized group of addresses. In addition, EIGRP implements modules forIP, Internetwork Packet Exchange (IPX), and AppleTalk, which are responsible for the protocol-specific routing tasks. This training module includes information about only the IP module.
   Typically, distance-vector protocols maintain the information about only one path to the destination, the best path. This information consists of total metric (distance) and the next hop (vector) to the destination. For example:
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   If a router, Router_A, learns about a destination (Network A) from two different routers, it chooses the best path by examining the total metric of each path. After it chooses the best path, it discards any information about the alternate (nonbest) path. In the above example, the best path would be through Router_C (assuming hop count as a metric). All information about the path through Router_B is discarded. If the path between Router_A and Network A is somehow broken, Router_A removes the route from its routing table after a certain amount of time, usually three update periods (in the case of Routing Information Protocol (RIP), this time would be 90 seconds, not including any hold-down timers). After this route is removed from the routing table, Router_A then learns about Network A via Router_B (because Router_B has been sending periodic updates). It could take from 90 to 120 seconds before Router_A installs the new route to Network A through Router_B.
   EIGRP, on the other hand, builds a topology table from information it learns from each of its neighbors. The information sent by EIGRP is nonperiodic and contains only new information. Using the Diffusing Update Algorithm (DUAL), EIGRP then chooses a best path (successor) and alternate loop-free paths (feasible successors) that allow for fast convergence. This information is kept in a topology table separate from the routing table. Upon losing a route to a destination, EIGRP looks for feasible successors in its topology table. If a feasible successor does exist, EIGRP begins using it immediately. If no feasible successors exist, EIGRP queries its neighbors.
   For all the above to be accomplished, the components of EIGRP must provide:
   
   • Reliable transport mechanism
   • Neighbor discovery/recovery process, which allows EIGRP routers to discover and track other EIGRP speaking routers that are on directly connected networks; part of this process must be done reliably (guaranteed)
   •  A way to discover which paths are loop free
   • A process to clear bad routes from the topology table of all routers on the network
   • A process for querying neighbors to find paths for lost destinations
   
   
   
   
3. EIGRP summary:
   - Reliable Transport protocol
   - Neighbor Discovery/Recovery and Updates
   - Metric
   - Determining if a Path is Loop Free
   - Clearing Bat Routes and Querying Neighbors
   - Address Summarization
   - Redistribution
   - Load Balancing
   - Administrative Distance (AD):

   Connected Route	0
   Static Route	1
   EIGRP Summary Route	5
   External BGP	20
   EIGRP Internal Route	90
   IGRP	100
   OSPF	110
   ISIS	115
   RIP	120
   EGP	140
   EIGRP External Route	170
   Internal BGP	200

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