From: Snow, Tim (timothy.snow@eds.com)
Date: Mon Dec 01 2003 - 21:31:02 GMT-3
Hopefully this well help, some of my eigrp notes when studying.
EIGRP uses a composite metric that is derived from the five submetrics. When
EIGRP computes the composite metric, it uses a formula that involves five
constants or "k" values. The constant values have default value such as the
following:
k1 = k3 = 1 and k2 = k4 = k5 = 0
By setting k2, k4, and k5 to 0, it essentially nullifies the submetrics of
load (k2) , reliability (k4) , and MTU (k5). This is precisely why you
should first use delay and then bandwidth when trying to influence which
routes EIGRP prefers. The formula EIGRP uses to calculate the composite
metric is as follows:
Metric = 256 x ([k1 x BWmim + (k2 x BWmim ) / (256-LOAD) + k3 x DELAYsum ]
x X)
where the following is true:
BWmim = 107 / bandwidth_of_slowest_link DELAYsum =
(delays_along_the_path) X = k5 / (reliability + k4) if and only if k1<>1,
if k1 = 1 then X = 1 With the k values set at the default value you have
k1 = k3 = 1 k2 = k4 = k5 = 0 CM = 256 x (BWmim + DELAYsum )
Using the default values of constants, k1 = k3 = 1 and k2 = k4 = k5 =0, the
formula quickly breaks down to this:
(256 x [BWmim and DELAYsum])
Substituting the constants, you have the following:
CM = 256 x ([1 x BWmim + (0 x BWmim) / (256-LOAD) + 1 x
DELAYsum] x 1) CM = 256 x ([BWmim + (0) / (256-LOAD) +
DELAYsum] x 1) CM = 256 x (BWmim + DELAYsum)
NOTE
For reference, the metric is computed the same way for IGRP, except the
result of bandwidth and delay is not multiplied by 256, and the DELAY sum
variable is divided by 10.
CM = (k1 x BWmin + [k2 x BWmin] / [256-LOAD] +
[k3 x DELAYsum] x X)
where the following is true:
BWmin = 107/ bandwidth_of_slowest_link DELAYsum
= S(delays_along_the_path) /10 X = k5 / (reliability + k4) if and only if
k1<>1, if k1=1 then
X=1 k1=k3=1 k2=k4=k5=0
With k values set at the default value, you have:
CM = BWmin + DELAYsum
In short, TOS is always 0, "metric weights TOS BANDWIDTH LOAD DELAY
RELIABILITY MTU"
Tim
#12042
-----Original Message-----
From: Stuart Reabow [mailto:stuartbr@hotmail.com]
Sent: Monday, December 01, 2003 3:16 PM
To: ccielab@groupstudy.com
Subject: EIGRP Weights
Hi,
Does anyone have any docs on modifying the EIGRP metric weights? I can't
find any details on the Cisco site.
Thanks
Stuart
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