Static Routing and Distance Vector Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 338

The default timers can be changed with the timers basic update invalid holddown flush
command. This is a subcommand under router igrp .

IGRP Metrics
IGRP uses a composite metric based on bandwidth, delay, load, and reliability. These metrics
are discussed in Chapter 2. By default, bandwidth and delay are used to calculate the composite
metric.

The composite metric is calculated as follows:

IGRP metric = {k1 × BW + [(k2 × BW)/(256 - load)] + k3 × delay} × {k5/(reliability + k4)}

In this formula, BW is the lowest interface bandwidth in the path and delay is the sum of all
outbound interface delays in the path. Reliability and load are measured dynamically by the
router. A 100 percent reliability is expressed as 255/255. Load is expressed as a fraction of 255.
An interface with no load is represented as 1/255.

By default, k1 and k3 are set to 1 and k2, k4, and k5 are set to 0. With the default values, the
metric becomes the following:

IGRP metric = {1 × BW + [(0 × BW)/(256 - load)] + 1 × delay} × {0/(reliability + 0)}

IGRP metric = BW + delay
The BW is 10,000,000 divided by the smallest of all the bandwidths (in kbps) from outgoing
interfaces to the destination. To find delay, add all the delays (in microseconds) from the
outgoing interfaces to the destination and divide by 10. (The delay is in 10s of microsecs.)

Example 7-17 shows the output interfaces to reach network 172.16.2.0 from Router9. The path
takes a serial link and the Ethernet interface of Router10. The bandwidths are 10,000 and 1544,
and the slowest bandwidth is 1544. The sum of delays is 20,000 + 1000 = 21,000.
Example 7-17 Output Interfaces to Reach Network 172.16.2.0 from Router9

Router9>show interface serial 0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 172.16.4.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Router10>sh int e 0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0010.7b80.bad5 (bia 0010.7b80.bad5)
Internet address is 172.16.2.1/24
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255