You have just been introduced to the Enhanced IGRP routing protocol.
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Enhanced IGRP allows for fast convergence. This is because it stores neighbors' routing information on which it can draw in the event of a topology change. Routes are recomputed only when this information reveals no alternate route.

Enhanced IGRP performs updates only when necessary. Only the updated information is sent,
and that information is sent only to routers that
require it.

The DUAL finite state machine is the process by which routes are determined for the transmission
of packets.

Enhanced IGRP currently supports three network layer protocols. These are TCP/IP, AppleTalk, and Novell IPX.

For each supported network layer protocol,
Enhanced IGRP maintains a set of three tables - a neighbor table, a topology table, and a routing table.

Enhanced IGRP routers multicast hello packets to make neighbors aware that the router is operating and reachable.

Enhanced IGRP broadcasts certain types of packets reliably. This requires the unicast of an acknowledgment packet by the receiving neighbor.

Update packets are reliably multicast in order to inform neighbors of changes in the transmitting router's routing table.

Query packets are multicast by routers attempting to establish a new route to a destination. Neighbors that receive query packets must respond by unicasting reply packets containing information as
to how they reach the destination.

Request packets are unicast when a router seeks specific information about route server applications.

Enhanced IGRP routers list their reachable neighbors in neighbor tables. A neighbor remains reachable provided it transmits a hello packet before the hold time associated with the previous hold packet expires.

Topology tables are used to store neighbors' routing tables. There is an entry for each destination, and a listing for each router with a path to that destination.

Provided that a successor can be found from the topology table, the destination entry remains
passive. Otherwise, the entry becomes active and query packets must be multicast in order to determine a successor.

Topology tables are used by DUAL to identify the successor with the best cost to a destination.

Enhanced IGRP stores the path it uses to each destination in protocol-dependent routing tables.

Enhanced IGRP routers can determine routes dynamically from routing table information supplied by neighbors, or routes can be set by network administrators based on local network policy.

When a topology change is required, faster convergence is provided if a successor can be identified from a topology table.
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You have been introduced to Enhanced IGRP on a network.
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Enhanced IGRP supports three network level routing protocols:

    • AppleTalk RTMP
    • IPX RIP
    • IP RIP

Enhanced IGRP uses composite metrics to determine the best path. Its metric structure is similar to IGRP's.

If you are migrating from IGRP to E-IGRP, route redistribution between routers within an AS is automatic, while route redistribution between ASs must be manually configured.

In order to enable Enhanced IGRP routing on a network you must:

    • enable the E-IGRP process at each router
    • define the directly connected networks with
      which each router interfaces
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You have seen how routing is monitored in the Enhanced IGRP process.
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There are several methods you can use to minimize routing updates

    • prevent updates being generated on the
      interface

    • eliminate routing protocols

    • filter routing updates

    • minimize route advertising with 
      redistribution

Route summarization procedures condense routing information.

Using summarization, routers can reduce some sets of routes to a single advertisement.

This reduces the load on the router and the perceived complexity of the network.

Without summarization each router in a network must retain a route to every subnet in the network.
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