Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 208 EIGRP Metrics EIGRP can utilize 5 separate metrics to determine the best route to a destination: • Bandwidth (K1) – Slowest link in the route path, measured in kilobits • Load (K2) – Cumulative load of all outgoing interfaces in the path, given as a fraction of 255 • Delay of the Line (K3) – Cumulative delay of all outgoing interfaces in the path in tens of microseconds • Reliability (K4) – Average reliability of all outgoing interfaces in the path, given as a fraction of 255 • MTU (K5) – The smallest Maximum Transmission Unit in the path. The MTU value is actually never used to calculate the metric By default, only Bandwidth and Delay of the Line are used. This is identical to IGRP, except that EIGRP provides a more granular metric by multiplying the bandwidth and delay by 256. Bandwidth and delay are determined by the interfaces that lead towards the destination network. By default, the full formula for determining the EIGRP metric is: [10000000/bandwidth + delay] * 256 The bandwidth value represents the link with the lowest bandwidth in the path, in kilobits. The delay is the total delay of all outgoing interfaces in the path. As indicated above, each metric is symbolized with a “K” and then a number. When configuring EIGRP metrics, we actually identify which metrics we want EIGRP to consider. Again, by default, only Bandwidth and Delay are considered. Thus, using on/off logic: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0 If all metrics were set to “on,” the full formula for determining the EIGRP metric would be: [K1 * bandwidth * 256 + (K2 * bandwidth) / (256 - load) + K3 * delay * 256] * [K5 / (reliability + K4)] Remember, the “K” value is either set to on (“1”) or off (“0”). * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 209 Configuring EIGRP Metrics EIGRP allows us to identify which metrics the protocol should consider, using the following commands: Router(config)# router eigrp 10 Router(config-router)# metric weights 0 1 1 1 0 0 The first command enables the EIGRP process for Autonomous System 10. The second actually identifies which EIGRP metrics to use. The first number (0) is for Type of Service, and should always be zero. The next numbers, in order, are K1 (1), K2 (1), K3 (1), K4 (0), and K5 (0). Thus, we are instructing EIGRP to use bandwidth, load, and delay to calculate the total metric, but not reliability or MTU. Our formula would thus be: [K1 * bandwidth * 256 + (K2 * bandwidth) / (256 - load) + K3 * delay * 256] The actual values of our metrics (such as bandwidth, delay, etc.) must be configured indirectly. We can adjust the bandwidth of an interface: Router(config)# int s0/0 Router(config-if)# bandwidth 64000 Router(config-if)# ip bandwidth-percent eigrp 10 30 However, this command does not actually dictate the physical speed of the interface. It merely controls how EIGRP considers this interface. Best practice is to set the bandwidth to the actual physical speed of the interface. The ip bandwidth-percent eigrp command limits the percentage of bandwidth EIGRP can use on an interface. The percentage is based on the configured bandwidth value. By default, EIGRP will use up to 50% of the bandwidth of an interface. The above command adjusts this to 30% for Autonomous System 10. If adjustments to the EIGRP metric need to be made, the delay metric (in tens of microseconds) on an interface should be used: Router(config)# int s0/0 Router(config-if)# delay 10000 Metric settings must be identical on the connecting interfaces of two routers; otherwise they will not form a neighbor relationship. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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