Certified Router Support Professional Best Route Calculation feasible distance and reported distance

Best Route Calculation feasible distance and reported distance

EIGRP uses a composite metric, calculated as a function of bandwidth and delay by default.  The calculation can also include interface load and interface reliability, although Cisco recommends against using either.  EIGRP calculates the metric for each possible route by inserting the values of the composite metric into a formula.

The formula, assuming that the default settings use just bandwidth and delay, is as follows:

Metric = (((10^7)/least-bandwidth) +cumulative-delay) * 256

Least-bandwidth represents the lowest-bandwidth link in the route, using a unit of kilobits/second.  If the slowest link in a route is a 10Mbps Ethernet link, the first part of the formula is 10^7/10^4, which equals 1000.  You use 10^4 in the formula because 10Mbps is equal to 10,000 kbps (10^4 kbps).  The cumulative-delay value used in the formula is the sum of all the delay values for all links in the route, with a unit of “tens of microseconds”.  You can set both bandwidth and delay for each link using the bandwidth and delay interface subcommands.

EIGRP updates list the subnet number and mask, along with the cumulative delay, minimum bandwidth, along with the other typically unused portions of the composite metric.  The router then considers the bandwidth and delay settings on the interface on which the update was received and calculates a new metric.

If the metric is a tie, by default a router would place up to four equal-metric routes into the routing table, sending some traffic over each route.

Feasible Distance and Reported Distance

* Feasible Distance (FD): The metric of the best route to reach a subnet, as calculated on a router.

* Reported Distance (RD): The metric as calculated on a neighboring router and then reported and learned in an EIGRP Update.

Caveats with Bandwidth on Serial Links

EIGRP’s robust metric gives it the ability to choose routes that include more router hops but with faster links.  To ensure that the right routes are chosen, engineers must take care to configure meaningful bandwidth and delay settings.  Serial links default to a bandwidth of 1544 and a delay of 20,000 microseconds.  IOS cannot automatically change the bandwidth and delay settings based on the Layer 1 speed of a serial link.  So, using default bandwidth settings on serial links can lead to problems.

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