Quick notes for EIGRP

  • EIGRP  is  a Cisco proprietary protocol which means it will work only on Cisco routers.
  • EIGRP is also called advanced distance vector or Hybrid routing protocol.
  • Multicast or unicast is used for exchange of information.
  • Multiple network layer protocols are supported.
  • 100% loop-free.
  • By default,EIGRP will limit itself to use no more than 50% of the interface bandwidth
  • Maximum Hop-Count is 255[ 100 by default]
  • EIGRP is classless protocol
  • EIGRP supports VLSM and authentication
  • EIGRP uses Diffusing Update Algorithm (DUAL) to determine the best path among all “feasible” paths. DUAL also helps ensure a loop-free routing environment.
  • EIGRP will form neighbor relationships with adjacent routers in the same Autonomous System (AS)
  • EIGRP uses RTP (Reliable Transport Protocol) and its function is to deliver EIGRP packets between neighbors in a reliable and ordered way.
  • All EIGRP routing information are exchanged between neighbors via multicast using  the address 224.0.0.10
  • EIGRP routers do not send periodic, full-table routing updates.  Updates are sent when a change occurs, and include only the change
  • K-Values are used for calculating metric. By default EIGRP consider k1 and k3 only
  • EIGRP having internal Administrative distance as 90 and external AD as 170
  • EIGRP summary route AD value is 5
  • In EIGRP Summarization is enabled by default.”No auto-summary” command is needed because by default EIGRP will behave like a classfull routing protocol which means it won’t advertise the subnet mask along the routing information. For example 1.1.1.0/24 and 2.2.2.0/24 will be advertised as 1.0.0.0/8 and 2.0.0.0/8. Disabling auto-summary will ensure EIGRP sends the subnet mask along.
  • EIGRP converges rapidly after link failure
  • EIGRP can load balance on both equal and unequal cost paths.”Variance”command is used to configure load balancing
  • EIGRP supports up to 6 loop-free paths with a metric lowers than the product of variance and the best metric
  • Manual summarization can be enabled at any interface
  • If a packet is not acknowledged,EIGRP re transmit the packet to the non responding neighbor as a unicast.No other traffic is sent to this neighbor until it responds.After 16 unacknowledged re-transmissions,the neighbor is removed from the neighbor table
  • The process of neighbor discovery and route exchange between two EIGRP router is as follows
    • Router A sends out a hello
    • Router B sends back a hello and an update.The update contains routing information
    • Router A acknowledges the update
    • Router A sends its update
    • Router B acknowledges
  •  A neighbor is considered as lost if no hello is relieved within 3 hello periods

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