EIGRP Tuning

EIGRP Tuning
The metrics used with EIGRP are tuned in the same manner as the metrics for IGRP. Metrics
are tuned to change the manner in which routes are calculated. The same command is used for
IGRP and EIGRP. In order to enter the following command, you must be in router configuration
mode:
metric weights tos K1 K2 K3 K4 K5
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
Enhanced Interior Gateway Routing Protocol 127
a weight, you are able to specify what is most important. If bandwidth is of greatest concern to a
network administrator, a greater weight should be assigned to K1. If delay is unacceptable, the K2
constant should be assigned a greater weight. The tos variable is the type of service. Refer back
to Table 4.2 for the relationship between the constant and the metric it affects. Also, remember
that EIGRP uses only bandwidth and delay, by default, when calculating routes.
Other tuning is possible. All routing protocols have an administrative distance associated
with the protocol type. If multiple protocols are running on one router, the administrative distance
value helps the router decide which path is best. The protocol with the lower administrative
distance will be chosen. EIGRP has a default administrative distance of 90 for internal
routes and 170 for external routes. Use the following command, in router configuration mode,
to make changes:
distance 1-255
Valid values for the administrative distance range from 1 to 255. Again, the lower the value,
the better. If an administrative distance of 255 is chosen, routes will be considered unreachable
and will be ignored.
When redistributing static routes or routes from other routing protocols into EIGRP, there are
two options on how you can enter the metric for the redistributed routes. The first option is to set
the metric for each instance of redistribution; that command will be covered in Chapter 10. The
second option is to set a default metric for all redistributed routes. This gives you less granularity
when setting metrics, but it is faster. The following command, when entered in router configuration
mode, sets the default metric:
default-metric bandwidth delay reliability load 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)
load = range from 0 to 255 (255 means the link is completely loaded)
MTU = a value between 0 and 4,294,967,295
EIGRP allows you to set the value for Hello intervals and holddown timers on a per-interface
basis. Remember, it’s safer to leave the default settings for the timers. Adjusting timers can cause
your internetwork to react in unexpected ways. In order to set the Hello interval, you must first
decide which interface to set it on. After navigating to interface configuration mode for the
selected interface, the following command needs to be entered:
ip hello-interval eigrp AS# seconds
AS# = the EIGRP autonomous system number
seconds = the amount of time, in seconds, for the Hello interval.
The default setting for the Hello interval is 60 seconds for low-speed NBMA networks and 5
seconds for all other networks.
The hold time is the amount of time a router will wait to receive a Hello packet before it marks
all of the routes from the neighbor as unavailable. The hold time default is three times the Hello
interval. As a rule of thumb, the hold time should always be set to three times the Hello interval.
128 Chapter 4  IGRP and EIGRP
As in setting the Hello interval, you must set it on an interface. After you have selected the interface
and navigated to interface configuration mode, enter the following command:
ip hold-time eigrp AS# seconds
AS# = the EIGRP autonomous system number
seconds = the amount of time, in seconds, for the hold time.
In this section, we explained EIGRP metrics and how to fine-tune EIGRP. We will now take
a look at redistribution for EIGRP at a high level.