EIGRP Basics

EIGRP is a Cisco proprietary protocol. If you don’t mind being locked in using a vendor specific protocol you will then enjoy using a robust, well-developed protocol that will meet most enterprise network needs. Here are a few basic bullet points about EIGPR:

  • EIGRP uses 3 steps to add routes to the global routing table
    • Neighbor Discovery – Initially when EIGRP is enabled, there are hello packets set between interfaces enabled for EIGRP. Those hello packets discover all the EIGRP neighbors that are participating in the establishing a neighborship.
    • Topology Exchange – Once neighbors are created, a full topology exchange occurs between the neighbors and the process of vetting out a successor/feasible successor begins.
    • Choosing Routes – When all the computations for the various networks are completed the best metric/path route is installed in the global routing table.
  • EIGRP Neighbors exchange a full routing table initially, thereafter only needed networks are re advertised, which will usually be due to a link, path, router being down, or if a new path is found with a better metric.
  • All EIGRP update messages are sent to multicast IP address of
  • EIGRP utilizes RTP to make sure hello/update messages are delivered
  • EIGRP uses a very specific algorithm to compute its “best routes”
    • EIGRP computation formula (below image was copied from wikipedia):
    • K values represent the components used to compute the metric
    • K1 – K5 are used within the formula
    • K values need to match between neighbors
    • By default, only bandwidth and delay to compute the best path metric

  • EIGRP Supports MD5 authentication

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