3.4 EIGRP Operation Overview

CCNP RS 300-101, Implementing EIGRP, Uncategorized
Hello.

There are three tables that EIGRP maintains:

  1. Neighbor Table – The table populates all known adjacencies to the router. It carries information about hold time, age, and RTP sequence number.
  2. Topology Table – Used by the DUAL FSM (Diffusing Update Algorithm Finite State Machine). Populate all route topology information advertised by known adjacencies. Includes metrics. This is the table used to determine successors and feasible successors.
  3. Routing Table – Populated by successors and feasible successors, is the table used to make routing decisions.

Remember that once an adjacency is formed, the first update packet (always reliable) is sent via Unicast (where all subsequent updates are sent via multicast). It has its initialization bit set when this is the case.

#show ip eigrp neighbors (command) – Here are the fields:

  • H – Handle, represents the order that the neighbors were added to the table
  • Address – the Primary address of the adjacent neighbor
  • Interface – The connected interface where the neighbor resides
  • Hold – The hold timer, by default it will always be less than 15. Good place to check for non-standard timers.
  • Uptime – the amount of time since this router has received an update from the neighbor
  • SRTT – Smooth Round Trip Time, the amount of time it takes for an update and acknowledgment sequence to complete with the neighbor. It has a special computation:

SRTT = (SRTT’ * 0.8) + (RTT * 0.2)

The SRTT value is equal to the last round trip time multiplied by 0.8, added to the current round-trip time multiplied by 0.2.

  • RTO – Measured in milliseconds, is the amount of time the router will wait before retransmitting a packet to the neighbor if no acknowledgement is received.
  • Q Cnt – Queue Count, the number of EIGRP packets that are in software pending transmission. This is never good, should always be zero.
  • Seq Num – The last observed sequence number for update, query, or reply for the neighbor.

Issues that prevent formation of EIGRP neighbor adjacencies:

  1. AS Number Mismatch – Adjacent routers will only listen for packets from matching AS numbers
  2. K-Value Mismatch – they need to match identically between two neighbors, so if you change on one router, you must change on all potential neighbors as well
  3. Uncommon Subnet – Incorrectly configured interfaces result in the adjacent interfaces belonging to disparate networks
  4. Passive Interfaces – A good practice to enable by default, but if you aren’t mindful, updates will not be sent out of passive interfaces you forget to enable
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