CCNP BSCI Notes – EIGRP Principles


Supports routed protocols like IP and IPv6 via protocol-dependent modules

Uses Reliable Transport Protocol (RTP, Cisco proprietary) for some traffic (updates, queries, and replies)

Uses hellos to identify/monitor neighbors

Uses the Diffusing Update Algorithm (DUAL) to select routes

is IP protocol 88.

EIGRP supports proportional unequal-cost load-balancing among feasible routes.

Packet types

         Hello – Identify neighbors, sent as periodic multicasts

         Update – Advertises routes, only sent when there is a change, multicast to

         Ack – Acknowledges receipt of an update

         Query – Used to query routes from neighbors (multicast; unicast attempted up to 16 times if multicast gets no response)

         Reply – Used to answer a query (unicast)

Metric calculation

Metric = 256 * (K1 * bandwidth + ( (K2 * bandwidth) / (256 – load) ) + K3 * delay) * ( K5 / (reliability + K4))

K values are used to distribute weight to different path aspects:

           bandwidth – Defined as 107 divided by the speed of the slowest link in the path, in Kbps

           load – 8-bit value, not considered by default

           reliability – 8-bit value, not considered by default

           delay – constant value associated with interface type; EIGRP uses the sum of all delays in the path

K defaults: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0

K values can be manipulated by an admin, but routers must have matching K values to become neighbors


        Advertised distance – Cost advertised by a neighbor to get to a destination

        Feasible distance – Advertised distance + cost get to that neighbor

The feasibility requirement states "if my neighbor’s advertised distance is less than my feasible distance, the path will be loop free."

        Successor – The neighbor with the best path

        Feasible successor – All other neighbors which meet the feasibility requirement

        Split-horizon – A network is not advertised on the link from which is learned.


When a router loses its successor and has no feasible successors, it will query all remaining neighbors for a new route. Queries are recursive and will be forwarded to other neighbors until either a route is found, or a summarization boundary is reached.

Stuck in Active (SIA) – Queries which do not return a route before the active timer expires (usually 3 minutes), the router is considered stuck in active mode.

EIGRP Tables

Neighbor table

Stores information about neighboring EIGRP routers:

         Network address (IP)

         Connected interface

         Holdtime – how long the router will wait to receive another HELLO before dropping the neighbor; default = 3 * hello timer

         Uptime – how long the neighborship has been established

         Sequence numbers

         Retransmission Timeout (RTO) – how long the router will wait for an ack before retransmitting the packet; calculated by SRTT

         Smooth Round Trip Time (SRTT) – time it takes for an ack to be received once a packet has been transmitted

         Queue count – number of packets waiting in queue; a high count indicates line congestion

Topology table

Holds all routes received from neighbors, is built from updates, calculated by DUAL, and contains all the information required by the routing table

Routing table

Route types:

          Internal – Paths directly within EIGRP

          Summary – Internal paths which have been summarized

          External – Routes redistributes into EIGRP

[Report Dead Link] Please leave a comment or send email to report dead links, so that we will update new links within 24 hours.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.