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  1. Overview of Enhanced Internal Gateway Routing Protocol (EIGRP)
  2. EIGRP Features
  3. EIGRP Operation
  4. EIGRP Message Types
  5. EIGRP Configuration Commands
  6. EIGRP Troubleshooting commands

Overview of EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) or Enhanced IGRP is a Cisco proprietary routing protocol utilizing the Diffusing Update Algorithm (DUAL). The DUAL algorithim was invented by Dr. J.J. Garcia-Luna Aceves of SRI International as an improvement to the IGRP routing protocol. EIGRP was designed to be interoperable with standard IGRP. EIGRP is a hybrid protocol as it incorporates features of a Distance Vector routing protocol and features of a Link State routing protocol. EIGRP is often used in Cisco-based networks running multiple network-layer protocols.

EIGRP can redistribute its routes (and metrics) intoother routing protocols and accepts redistribution from other routing protocols as well.

EIGRP Features

  • Hybrid Distance Vector/Link State algorithm
  • Supports VLSM (subnets/supernets)
  • Integrates seamlessly with IGRP
    • Automatic Redistribution of Routes (IGRP <-> EIGRP)
    • EIGRP metrics are 256 times the IGRP metric and therefore 'directly translatable'
  • Fast convergence
  • Performs Partial Updates as needed
  • Consumes less bandwidth (no broadcasts, no periodic updates, updates contain only changes)
  • Supports multiple network layer protocols
    • Appletalk
    • Internet Protocol (IP)
    • Novell Netware (IPX/SPX)

EIGRP Operation

  • Four EIGRP Components
    • Neighbor Discovery/Recovery
      • Dynamically find other routers running IGRP/EIGRP
      • Dynamically forms neighbor relationships
      • Discover neighbor state (unreachable or inoperative) - Uses HELLO packets
    • Reliable Transport Protocol - Utilizes Reliable Transport Protocol (RTP) for delivery of EIGRP packets.
    • DUAL Finite State Machine
    • Protocol Dependant Modules
  • EIGRP tracks all routes advertised by all neighbors (feasible successor routes)
    • Selects best path
    • Selects a feasible successor route
      • CISCO.COM: A feasible successor is a neighboring router used for packet forwarding that is a least-cost path to a destination that is guaranteed not to be part of a routing loop
    • If no feasible successor exists,
      • queries are sent out to the network
      • Diffusing computation is performed to select another feasible route
      • Diffusing computation is not processor intensive (but affects convergence time
  • Maintains a copy of each neighbor's route table.
  • Sends updates only when changes in the metrics occur
  • Sends only the changes
  • Sends changes only to neighbors that need the information (no broadcasts)
  • Can redistribute routes from RTMP, OSPF, RIP, IPX RIP/SAP, IS-IS, EGP and BGP
  • Routing
    • Administrative Distance for EIGRP
      • Summary Routes [5]
      • Internal Routes [90]
      • External Routes [170]
    • Neighbor Tables
      • Neighbors recorded (IP and interface of neighbor)
      • One neighbor table for each network protocol
      • Neighbors send hold time in HELLO packet
      • Hello packet contains hold down time
      • If neighbor is not heard from within the hold down time, topology table is changed via DUAL
      • Contains RTP information (Sequence Number, transmission list of packets, round trip timers optimize retransmission interval).
    • Topology Tables
      • Contains all destinations advertised by all neighboring routers
      • Each topology table entry contains:
        • destination address
        • list of neighbors used to reach the destination
        • for each neighbor store the advertised metric for each destination
        • Best Path = sum of best advertised metric from all neighbors and the link cost to the best neighbo
    • Feasible Successors
      • Route(s) inserted by EIGRP into the routing table will have the best metric of all the routes in the table.
      • Any route to a destination whose metric is less than the current entry or entries in the routing table is a feasible successor.
      • When the current route enters the 'active' state, the feasible successor is inserted in the routing table.
      • The list of feasible successors may have to be re-evaluated if a neighbor sends a topology change or updates the metric to a destination.
      • If a neighbor who is the only feasible successor to a destination goes down, all of the neighbor's routes enter the active state and trigger route recomputation.
    • Route States (two states)
      • Active - recomputation is being performed
      • Passive - no recomputation going on
      • If feasible successors are always available, a destination never goes into the active state.
      • Recomputation occurs when no feasible successor route exists
      • If a neighbor who is the only feasible successor to a destination goes down, all of the neighbor's routes enter the active state and trigger route recomputation.
      • Recomputation Process
        • Send a query packet to all neighboring routers
        • Neighbor sends
          • a reply that it has a feasible successor, or
          • a query packet to indicate it is partcipating in the recomputation
        • Routes in the active state cannot have their routing table information changed
        • Once all neighbors have replied the topology table entry for the destination returns to the pasive state and the router may then select a feasible successor.
    • Route Tagging
      • Internal routes come from neighbors with the same (E)IGRP AS number or from directly attached interfaces over which IGRP or EIGRP runs.
      • External routes come from other routing protocols or from static routes and are tagged with the following information:
        • Router ID of the router that distributed the route
        • AS number of the destination
        • Configurable administrator tag
        • ID of the external protocol
        • Metric from the external protocol
        • Bit flags for default routing

EIGRP Message Types

Type Transmit Sent Function


Multicast   Hello messages are used for neighbor discovery and neighbor recovery. If a hello message is not received within the configured interval, all neighbor entries are removed from the routing table and feasible successor routes re utilized.
  Unicast Reliably Hello messages are also used to acknowledge receipt of information. Zero byte acknowledgement (with ACK number)


Unicast Reliably Neighbor discovery
  Multicast Reliably Link cost or metric change updates
Queries Multicast Reliably Sent when one or more destinations enter the active state.
Replies Unicast Reliably Sent to originator of a query.
Requests Multicast or Unicast Unreliably Request specific information from neighbors


EIGRP Configuration

Basic EIGRP router configuration (Cisco)

router(config)# router eigrp <AS number>
Enable EIGRP routing and set the Autonomous System number.
router(config-router)# network
Configure the directly connected networks that will be advertised.

EIGRP Troubleshooting

show ip eigrp topology
Shows only feasible successor routes
show ip eigrp topology <network> 
Shows all entries in the topology table for the given destination network.
show ip eigrp topology all-links 
Shows all entries in the topology table
show ip eigrp topology [active | pending | zero successors ]
Show destinations that are in the active or pending states or have zero successors.




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