Unlike other well known routing protocols, such as RIP, EIGRP only sends incremental updates, reducing the workload on the router and the amount of data that needs to be transmitted.
One of the major reasons for this was the change to classless IPv4 addresses in the Internet Protocol, which IGRP could not support.
This eases the workload on a network administrator who does not have to configure changes to the routing table manually.
If the network changes (for example, a physical link fails or is disconnected), the path will become unavailable.
EIGRP is designed to detect these changes and will attempt to find a new path to the destination.
EIGRP is a distance vector & Link State routing protocol that uses the diffusing update algorithm (DUAL) (based on work from SRI International) to improve the efficiency of the protocol and to help prevent calculation errors when attempting to determine the best path to a remote network.
EIGRP determines the value of the path using five metrics: bandwidth, load, delay, reliability and MTU.
[3] EIGRP uses five different messages to communicate with its neighbor routers – Hello, Update, Query, Reply, and Acknowledgement.
EIGRP routing information, that has come from an EIGRP-enabled router outside the autonomous system, has a default administrative distance of 170.
[8][9] The Reliable Transport Protocol also contains other mechanisms to maximize efficiency and support multicasting.
For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric: where the various constants (
Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops may result.
The interface delay is a configurable static parameter expressed in tens of microseconds.
This way, redistributing information between EIGRP and IGRP involves simply dividing or multiplying the metric value by a factor of 256, which is done automatically.
A feasible successor for a particular destination is a next hop router that is guaranteed not to be a part of a routing loop.
A feasible successor provides a working route to the same destination, although with a higher distance.
The destination changes back from active to passive when the router received replies to all queries it has sent to its neighbors.
However, such neighbors do not provide the shortest path to a destination, therefore, not using them does not present any significant impairment of the network functionality.
These neighbors will be re-evaluated for possible usage if the router transitions to Active state for that destination.
A multiplier, called variance, is used to determine which paths to include into load balancing.
The variance is set to 1 by default, which means load balancing on equal cost paths.
Regarding the Unequal Path Cost Load Balancing function on industry usage, the network design can be flexible with the traffic management.
Cisco released details of the proprietary EIGRP routing protocol in an RFC in an effort to assist companies whose networks operate in a multi-vendor environment.
[1][15] Cisco has stated that EIGRP is an open standard but they leave out several core details in the RFC definition which makes interoperability hard to set up between different vendors' routers when the protocol is used.
[16] As of 2022 EIGRP has alpha support in FRRouting[17][18] and it seems to be generally unsupported by other routing software.