EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) is still a part of CCNP/CCIE curriculum so I am using this post for exam preparation or trouble shooting purposes.  Believe it or not you can still see this protocol being in use in commercial environments but it is not part of the new Cisco CCNA exam.

About EIGRP:

-          EIGRP replaced IGRP in 1993, initially Cisco proprietary now partially open standard 

        described in RFC7868 classless routing protocol

-          Hybrid routing protocol/Advanced distant routing protocol

-          uses DUAL algorithm (Diffusing update algorithm)

-          uses protocol number 88

-          protocol independent, uses multicast address 224.0.0.10 and FF02::A for IPv6

-          Supports unequal load balancing (Advantage over link-state protocols)

-          provides fast convergence (Advantage over link-state protocols)

EIGRP terminology:

-          Successor – next-hop router with the lowest path metric

-          Feasible Successor (FS) – backup route to the destination

-          Reported Distance (RD) – cost between next hop and a destination

-          Feasible distance (FD) – cost to the destination (cost to the next hop + RD)

-          Active route – route for which Query process is being used to find a loop free alternative route (i.e. when the link goes down)

-          Passive route – route that is ready to be used (no longer calculated by DUAL)

EIGRP Tables:

-          Neighbor Table – lists of all neighbors

-          Topology Table – list of all prefixes

-          Routing Table – best routes (Successors)

EIGRP Packet types:

   Uses RTP (Reliable Transport Protocol) to exchange information

-          Hello – forms relationship / keep alive

-          Update – updates can be partial or full routing table (only sent when forming neighbour)

-          Query – i.e. query is send when there is no Feasible Successor (FS)

-          Replay – response to the query

-          Ack – acknowledgment to the update, query or reply messages

EIGRP Metric calculation

-          by default Bandwidth and Delay are being used, K1 and K3 both have the value of 1

-          by default K2, K4 and K5 are set to 0

-          latency is measured in microseconds, reference bandwidth 10Gbps

EIGRP Wide metrics

-          was implemented to help with higher-capacity links(up to 655Tbps), K6 value was added

-          by default K6 is 0 which makes it backward compatible

-          latency is measured in picoseconds

EIGRP troubleshooting (things to remember)

-          by default hello timer is 5 seconds (60sec on older slower links)

-          hold timer is 3x Hello timer

-          to form neighbor relationship EIGRP ASN numbers must match, K values must match, if authentication is used password and keys must match

-          when router detects the loss of Successor, Feasible Successor is being used and the Update is being send and every router that receive it will run DUAL

-          when router detects the loss of Successor and there is no Feasible Successor, route state changes from passive to active and DUAL will send a Query to EIGRP neighbors

Stuck in active (SIA) – this happens when there is no reply within 3 minutes often caused by high CPU and memory usage, packet drop or unidirectional links.