EIGRP Overview

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco Proprietary, Advance Distance Vector, Classless Routing Protocol that uses a complex metric based on bandwidth and delay. The following are some features of EIGRP:

• Fast Convergance
• Support for VLSM
• Partial updates conserve network bandwidth.
• Support for IP, AppleTalk, and IPX.
• Runs directly over IP, using protocol number 88.
• Support for all layer 2 (Data Link Layer) protocols and Topologies.
• Sophisticated Metric that supports load-balancing across unequal-cost  paths.
• Use of Multicast (and unicast where appropriate) instead of broadcasts.
• Support of Authentication.
• Manual Summarization at any interface.
• Use Multicast 224.0.0.10.

EIGRP's function is controlled by four key technologies:

• Neighbor Discovery and Maintenance: Periodic Hello Messages
• The Reliable Transport Protocol (RTP): Controls sending, tracking, and acknowledging EIGRP messages
• Diffusing Update Algorithm (DUAL): Determines the best loop-free route.
• Protocol-independent Modules (PDM):  Modules are "plug-ins" for IP, IPX, and AppleTalk versions of EIGRP

EIGRP uses three tables:

•  The neighbor table is built from EIGRP hellos and used for reliable delivery.
• The topology table contains EIGRP routing information for best paths and loop-free alternatives.
•  EIGRP places best routes from its topology table into the common routing table.