Metric Association of K-Values

TABLE 4 . 2 Metric Association of K-Values
Value Metric
K1 Bandwidth
K2 Loading of the link (effective bandwidth percentage used)
K3 Delay
K4, K5 Reliability
Each constant is used to assign a weight to a specific variable. This means that when the metric
is calculated, the algorithm will assign a greater importance to the specified metric. By assigning
a weight, you are able to specify what is more important. If bandwidth were of greatest concern
to a network administrator, then a greater weight would be assigned to K1. If delay is unacceptable,
then the K3 value should be assigned a greater weight.
Cisco suggests leaving all K-values at their default, due to the likelihood of
unexpected results that might compromise your network’s stability. Consider,
for instance, the case where you might be inclined to adjust the default value
of K2 in order to include load as a variable included in routing decisions.
Because load is updated every five seconds, based on a rolling five-minute
average, the IGRP metric is now subject to change every five seconds. This can
cause links over which a new application process is initiated to go into holddown
due to the increased load causing the metric to rise more than 110 percent.
Conversely, a plummeting load can initiate flash updates to neighbors. As
you can see, either scenario is unacceptable and leads directly to route instability.
Take Cisco’s advice—leave these constants at their defaults.

Note that MTU is not actually represented by any of the K-value constants. This
is because MTU is never used in the calculation of the composite metric. The
use of MTU is discussed more in the EIGRP section later in this chapter.
As well as tuning the actual metric weights, you can do other tuning. All routing protocols
have an administrative distance associated with the protocol type. If multiple protocols are running
on a router, the administrative distance value helps the router decide which route was
learned by the most trusted method. The route learned by the method with the lowest administrative
distance will be chosen. IGRP has a default administrative distance of 100. The tuning
of this value is accomplished with the distance command, like this:
Router(config-router)#distance ?
<1-255> Administrative distance
Valid values for the administrative distance range from 1 to 255; the lower the value, the better.
A route with an administrative distance of 255 will not be used.
When redistributing static routes or other protocol types within IGRP, there are two options
on how you can enter the metric for the redistributed routes. The first option you have is to set
the metric for each instance of redistribution; the command will be covered in Chapter 10,
“Route Optimization.” The second option is to set a default metric for all redistributed routes.
This gives you less granularity when setting metrics but is faster to configure. The following
command, when entered in router configuration mode, will set the default metric:
default-metric bandwidth delay reliability loading MTU
bandwidth = a value between 0 and 4,294,967,295 (in Kbps)
delay = a value between 0 and 4,294,967,295 (in 10-microsecond units)
reliability = a range from 0 to 255 (255 is the most reliable)
loading = range from 0 to 255 (255 means the link is completely loaded)
MTU = a value between 0 and 4,294,967,295
The bandwidth and delay that you enter should be “ready to use.” This means
no adjustment will be made before using them in the metric calculation formula.
Bandwidth is expected to be the minimum bandwidth of all the links
along the path to the destination network, and the value you enter for delay is
considered to be already one-tenth the actual cumulative delay along the path
to the destination.
When a router receives multiple routes for a specific network, one of the routes must be
chosen as the best route from all of the advertisements. While the router is told that it’s
possible to get to a given network over multiple interfaces, it forgets about all but the best
and all traffic will be sent over the best route.