EIGRP II: Metric Calculation with a GPS…

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

2 Responses to “EIGRP II: Metric Calculation with a GPS…”

  1. well done, its great stuff . hope if you have time to rearange based on what is more important in the industry ….

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