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EIGRP Characteristics

  • Enhanced Interior Gateway Routing Protocol

  • EIGRP is resulting from IGRP

  • EIGRP was a Cisco proprietary protocol initially but now it is an open standard Hybrid routing protocol.

  • Also called advance distance vector protocol

  • Classless:

  • AD Value: Internal EIGRP 90, External EIGRP 170

  • Uses bandwidth, delay, load, reliability as metric; Bandwidth and Delay by default

  • Multi protocol support: IPv4, IPv6, IPX, Apple Talk

  • Uses hello packets to discover the neighbors

  • EIGRP uses the DUAL (Defusing Update Algorithm)

EIGPR DUAL

  • DUAL is used to:
    • Determine best path to a destination
    • Find out loop-free backup routes
    • Facilitates faster convergence
  • EIGRP takes 3 steps for network convergence:
    • Forms neighbor relationship
    • Exchange of topology information
    • Routing table propagation using DUAL
  • Auto-summary is enabled by default.
  • Purpose of DUAL
    • To prevent routing loops loop-free backup route, faster convergence
  • Successor
    • Primary route to a destination
  • Feasible successor
    • Backup route to a destination

AS Number & RTP

  • All routers who are supposed to exchange routing information must have the same AS (Autonomous Number) number
  • All routers and networks under a single authority uses the same AS number that ranges from 1 - 65535
  • EIGRP uses RTP (Reliable Transport Protocol) for exchange of routing information
  • RTP supports both reliable and unreliable updates
  • EIGRP message types: Hello Packet, Query Packet, Update packets, acknowledgement packets, reply packets.

EIGRP Neighborship

  • For two routers to become adjacent or neighbors:
    • Active hello
    • AS number matched
    • K values matched (#show interfaces)
    • Authentication matched
  • K values are:
    • Bandwidth (default: k1)
    • Delay (default: k2)
    • Reliability (default: k3)
    • Load (default: k4)
    • MTU (not used: k5)

Troubleshooting Tip

  • If two routers fail to become neighbors, one of the followings can be the reason:
    • Mismatched EIGRP Authentication Parameters (if configured)
    • Mismatched EIGRP K values (Metric)
    • Mismatched EIGRP Autonomous System (AS) Number
    • The neighbors are not on a common subnet
    • The hello interval is short and expires before it reaches to the neighbor

EIGRP Updates

  • Sends complete routing table for the first time and later on use partial and bounded updates
  • Non-periodic updates; minimizes bandwidth utilization
  • Updates can be sent as a unicast or multicast 224.0.0.10
  • Uses RTP for exchange of routing updates