Cisco bandwidth statement

IGRP, EIGRP, and OSPF all use the bandwidth statement.
TCP will also adjust its initial retransmission parameters based on the bandwidth configured on the interface.

OSPF uses cost as its routing metric, which it calculates using bandwidth.

OSPF takes 108 and divides it by the bandwidth of the interface. To calculate the cost of a full T1, OSPF divides 100,000,000 by 1,544,000, which returns an OSPF cost of 64.
(Cisco routers don't use floating-point math, so they drop the numbers after the decimal.)

On the other hand, EIGRP uses the bandwidth of the link to calculate its routing metric.
EIGRP metric formula:

metric = [K1*bandwidth + (K2*bandwidth)/(256 - load)
+ K3*delay] * [K5/(reliability + K4)]

Example:

interface Serial0/0
bandwidth 128
ip address X.X.X.X 255.255.255.0

This command has only one option—the bandwidth, in kilobits, of the interface.

Router(config-if)# bandwidth ?
<1-10000000> Bandwidth in kilobits
Router(config-if)#bandwidth

There are always default bandwidth values set for each type of interface, such as the Serial
interface:

Router# show interface s0/0
Serial0/0 is administratively down, line protocol is down
Hardware is PowerQUICC Serial
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

In the case of a serial interface, the default bandwidth is 1,544 K (or a full T1 circuit).
However, you could have a fractional T1 circuit, and the default may be incorrect.