Archive for delay

EIGRP II: Metric Calculation with a GPS…

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 6, 2012 by jjrinehart

GPS via Google Maps

Unlike most of you, I can get lost in my own backyard; imagine how misdirected I can get when I am actually driving!  My beautiful wife Brenda (a feisty little redhead) is a walking, talking, nearly-always-right human GPS, which is wonderful except when I am driving alone and trying to get somewhere.  Fortunately, in our world of GPS devices and smartphones (complete with Google Maps), I have some recourse for not getting lost.  Even so, these devices are not foolproof, as they once told me that a hotel was in the middle of the Potomac River in Washington DC!

These handy little devices are great because they rely on large databases which contain information on mileage, geography, construction, road conditions, etc., when trying to help get you from one place to another.  Just as GPS devices use multiple criteria for recommended a route of travel, so EIGRP relies on several different elements in calculating metric for destination networks.  This is remarkably different from every other interior routing protocol, which typically relies on a single element for its metric.  Here is a breakdown of the five elements of the EIGRP metric:

1. Bandwidth: At first glance you might think that this is identical to the OSPF cost concept, but there are a couple of important differences.  While bandwidth does create a cost-like factor (the higher the better), in EIGRP this cost is not cumulative.  Instead, it is based on the lowest bandwidth along the path to a destination network.  For example, if one path has all 100 Mbps links and another has 100 Mbps links with one 10 Mbps links, the first path will be preferred because the smallest (called constrained) bandwidth is 10 Mbps.  This might sound less ideal than a cumulative cost until you think of how backed up a highway gets when narrowed down to one or two lanes!

2. Delay: Unlike bandwidth, this factor is cumulative along the entire path.  The greater the delay, the less desirable the route is because delay is caused by lower bandwidth and/or congestion.  Why choose a 4 lane superhighway if the traffic is crawling along at a very slow speed?

3. Reliability: As the name implies, this measures how reliable the route is (0-255)

4. Load: How loaded or saturated the route is (0-255)

5. MTU: The IPv4 Maximum Transmission Unit size.

Keep in mind that only bandwidth and delay are enabled by default for metric calculation, and each element is called a K-Value.  Always make sure the K-Values match between neighbors or a relationship will never form.

Next time we will key in on route selection in EIGRP…

– Joe

EIGRP I: The “Borg” of IP Networking

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 3, 2012 by jjrinehart

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