EIGRP I: The “Borg” of IP Networking

Resistance is Futile!

While not completely universal, there often seems to be a natural affinity between networking geeks and the Star Trek science fiction franchise.  While I grew up watching reruns of the original series, I took a liking to the characters of the “next generation” cast (for you purists, TOS and TNG respectively), and found the Borg to be the most compelling super villains ever!  For those unfamiliar with these mechanical zombies, they are mechanically augmented humanoids all linked by technology to a central mind, being partly biological and partly machine.  Think of Darth Vader as “Borg Lite!”

The complexity of Borg characters is that they are not completely humanoid, though possessing certain biological characteristics, nor completely mechanical, though having aspects of that as well.  In essence, they represent a hybrid of the two, and it is worth noting that the term used for this type of being generally is a cyborg.  In other words, they are not one or the other, but a blend of both.  This is precisely the situation in which EIGRP finds itself, having characteristics of Distance Vector Protocols, as well as features of Link-State Protocols.

When you read white papers, books, and documentation on EIGRP, you will notice this type of duality present in characteristics of EIGRP.  For example, like LS protocols, EIGRP build formal neighbor relationships and tracks the state of those relationships.  Conversely, this protocol also uses the familiar DV loop prevention mechanisms such as split-horizon and hold-down states.  When you peruse the literature on EIGRP you will typically hear the word hybrid to reflect the nature of operations, although I have seem references to balanced-hybrid and advanced distance-vector as well.

A few similarities exist between EIGRP and OSPF, beyond neighbor relationships alone.  Although not a Link-State Database, EIGRP does build its own table of subnets called the Topology Database (more on this in another entry).  It also chooses a lowest-cost route to a destination subnet, but the criteria are entirely different from OSPF.

The differences between the protocols are more numerous that the similarities.  First, OSPF is based on an open standard, while EIGRP is Cisco proprietary.  In short, if you have non-Cisco devices in your network, you either have to do some form of redistribution (sharing routes between protocols) or you have to use OSPF or another standardized protocol.  Another significant difference is that EIGRP is not formal—no areas, DR’s/BDR’s, and so forth; no hierarchy exists, which certainly makes it simpler in many respects.

Next time we will dig into the EIGRP metric and route selection process.

– Joe

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