Is EIGRP a Hybrid Routing Protocol or Advanced Distance Vector Routing Protocol?
This question was discussed recently on the Cisco CCIE Routing and Switching Study Group website. It took me years to work out that the problem with this question is the question itself, not the answer.
The question gives the impression that “Hybrid” and “Distance Vector” (or “Advanced Distance Vector“) are mutually exclusive. Like “Is this animal a dog or a cat?” In fact, it is more like “Is this animal a mammal or a dog“. If you are looking at a tiger, then clearly the answer is “mammal”. If you are looking at a wolf however… it is a little tricky.
In my simple analogy above, EIGRP is a wolf. There is no doubt that it is a Distance Vector protocol, because it records only the distance to destinations – not the path (as OSPF and ISIS do). Although I think you would be hard pressed to find any official Cisco documentation calling it a Distance Vector Routing Protocol without the wordAdvanced in front of it, to distinguish it from its predecessor IGRP. This is partly because there are some limitations with other Distance Vector protocols (especially IGRP) that made Cisco want to avoid the Distance Vector label.
However, because it also maintains some sense of State with its neighbours by sending Hellos, it also has some of the characteristics of a Link State Routing protocol, so in an effort to even distance itself even further from the Distance Vector label, Cisco often refers to EIGRP as a Hybrid Routing Protocol, but this is best thought of as a marketing term, because it tells you nothing about the nature of the protocol itself – Hybrid? Hybrid of WHAT exactly? Hybrid of a wolf and a dog????
So the most correct way to describe EIGRP is indeed as an Advanced Distance Vector Routing Protocol. It uses the Diffusing Update Algorithm, (DUAL) which is different to the Bellman-Ford algorithm used by RIP and different to Dijkstra’s Shortest Path First algorithm used by OSPF and IS-IS.
[Originally posted by me at https://learningnetwork.cisco.com/message/327441#327441]