Here you are my fast notes regarding EIGRP.

  • IP Protocol: 88, Uses Multicast IP:
  • Protocol Dependent Modules (IP, IPX, Appletalk)

Determining Loop Free Path

  • Feasibility Condition (AD<FD) must be meet
  • Split-Horizon – never advertise a route out of the interface through which you learned it

Reliable Transport Protocol (RTP)

  • Packets (reliable delivery and packets will be delivered in order – waits for ACK)

Guaranteed delivery > reliable multicast and confirmation reply as unicast ACK

Ordered delivery > 2 sequence number in EIGRP packet (incremented seq each pack. and last received seq)

  • HELLO – multicast, unreliable
  • ACK – (it Hello packet with no data in them), unicast, unreliable
  • UPDATE – include route info, multicast/unicast, reliable
  • QUERY – manage DUAL computation, multicast or unicast, reliable
  • REPLAY – manage DUAL computation, unicast, reliable
  • If packet is reliable/multicast and ACK is no received from the neighbor
    • Then packet is retransmitted as a unicast to unresponding neighbor
    • If ACK is not received after 16 unicast retransmission > neighbor is dead
    • Timers – calculated based on the Smooth Round Trip Time (SRTT)>average time between transmission of packet to the neighbor and the receipt of an ACK
    • Multicast Flow Timer – time to wait for ACK before switching from multicast to unicast
    • Retransmission Timeout (RTO) – time between subsequent unicast packets

Neighbor Discovery/Recovery

  • Hello – 5/60 seconds – ip hello-interval eigrp
  • Hold-Time – 15/180 – ip hold-time eigrp

DUAL – Diffusing Update Algorithm

  • Feasible Distance – lowest calculated metric to the destination
  • Successor – router (next-hop) with the lowest (best) metric to the destination
  • Feasible Successor – one of the backup of Successor that meets Feasibility Condition

Before DUAL compute the metric following have to take place

  1. Establish adjacency between neighbors
  2. Updates exchange
  3. DUAL calculates metric based on the received Advertised Distance from the neighbor + cost to the neighbor
  4. Lowest calculated metric is Feasible Distance (FD), router that advertised this metric is Successor
  5. Successor’s router with the best metric is RIB
  6. If the Feasibility Condition is meet when AD<FD (FD of current successor) [Loop Free condition]
  7. If neighbor’s AD to the destination meets the FC neighbors become a Feasible Successor. FS can be elected as Successor when current Successor goes down and if has the lowest metric to the destination with comparison with other Feasible Successors

DUAL Finite State Machine

  1. If FS can’t be found in the Topology Table, then router begins a Diffusing computation and route is Active
  2. Router sends Query to all of it’s neighbors
  3. If Neighbor has one or more Feasible Successors for the destination it will send reply to the questioning router
  4. If Router doesn’t receive reply to query in Active time, route is declared Stuck-In-Active (SIA)
  5. Neighbors that didn’t reply will be removed from the neighbor table


METRIC (BW, Delay, Load, Reliability)

M = (10 000 000 /minBW + DLY/10 [in 10 micro sec])*256

traffic-share balanced

  • Bandwidth— A value represented as the smallest bandwidth between the source and destination
  • Delay— The collective delay of interfaces along the path
  • Reliability— The lowest (worst) reliability along the network path
  • Load— Represented by the worst load on a connection between the source and destination, in bps
  • MTU— The smallest maximum transmission unit value in the path
  • K1 = bandwidth
  • K2 = load
  • K3 = delay
  • K4 = reliability
  • K5 = MTU

Default EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

Stuck-In-Active (SIA)

  • timers active-time 3
  • timers active-time disabled
  • Stub and Summarization allows to reduce sending of Query to neighbors
  • show ip eigrp topology all-links – shows routes that are not Feasible Successors
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