EIGRP Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.



  • Best practice- always do no auto summary. Doing manual summarisation (done via interface) does NOT disable auto summary. Also manual summary creates null0 on the router creating the summary. ASBR best place to do manual summarisation.
  • When a successor route is down, the feasible route becomes the successor. The next route becomes Feasible route only is it meeting the AD is lower than FD successor rule
  • If no feasible successor, the route is marked as Active (unusable) & sends a query packet to neighbours for a route to destination.
  • multicast address & IP protocol number 88 & uses RTP for reliability.
  • Neighbours do not need to agree on hello & holdtime timers unlike OSPF. They need to agree on K values / metric
  • Can form adjacency on secondary addresses, but shows the primary add under show neighbours but cannot form if between a secondary interface to a primary that not on a common subnet
  • Change metric  under routing process not interface
  • D in routing table & AD = 90
    • D EX is for redistributed routes AD = 170
    • Summary route AD=5
    • Feasible successor = Backup route kept in Topology table
    • Successors = Primary route and kept in Routing and Topology table
    • 3 x tables = Routing, Topology and Neighbour
    • AD = Distance from neighbour router to destination.
    • FD = Distance from local router to Destination (FD = FD+AD)
    • Routing loop prevention = AD must be lower than FD of the successor route
    • 5 K weights is Bandwidth & delay (defaults), load, reliability and MTU
    • Equal cost load sharing = 4 paths by default. 6 maximum.
    • 5 Packets
      • Unreliable (don’t use RTP)
        • Hello – To form neighbours on & keep neighbour relationships alive. Sent every 5 seconds & 60 seconds on slow links (less than  T1)
        • Ack – acknowledges hello packets via unicast
  • Reliable (uses RTP)
    • Update – sent when changes occur in network & also for new routers to build routing and topology table
    • Query – sent when router loses successor route & has no feasible route.
    • Reply – Responds to query packets via unicast
    • Stuck in Active – loses successor and no feasible successor, so sends query. The neighbouring router does not respond to query and is therefore stuck in active SIA. SIA can happen if link is unidirectional (rare) or CPU to high to ack, or memory corrupt, or a bad link. Commonly to busy to answer due to high CPU or not enough memory
    • Variance command – For unequal cost load balancing. Done under routing process. Do lowest variance possible, as to high will load balance on undesirable routes. Once variance is done, routes will be seen in routing table e.g. feasible is now one of the successors. Clear ip route * to see in routing table.
      • Very important – No matter what the variance is, routes with AD higher than the successor FD will not be used
      • CIRS / Frame cloud
        • EIGRP uses 50% of bandwidth by default
        • No Subinterfaces, ALL VCs are equal = Add all spoke CIR
        • No Subinterfaces, ALL VCs are not equal = take the lowest CIR x no. Of spokes
          • Best practice for unequal spokes to make them point to point
  • Multipoint Subinterfaces, VCs are not equal = Add all spoke CIR
  • STUB – done in hub and spoke. Spoke no need to keep full routing as next hop is always hub. There for make as stub. Spoke not queried for routes. Has Default route to hub and is always neighbour with only a hub. Makes it very efficient and good for less powerful routers. Never has SIA issues as it not queried for successor routes etc. Stub only tells hub about connected & summary routes only. Won’t work if spokes are interconnected


  • Show ip eigrp traffic
    • Shows sent/received of all packets as well as SIA.
    • Show ip eigrp neighbour
      • Shows neighbours, interface, uptime
      • Debug eigrp packets
        • Debugs all packets. Shows what packet received / sent on what interface
        • Shows stuff like k-value mismatch
        • Debug eigrp neighbours
          • Says neighbours not found, or shows new peers, hold time expired etc
          • Eigrp stub
            • Makes it stub. Do under routing process.

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