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the router's serial interfaces. It is important to change the bandwidth of an
interface if you are using a routing protocol that uses the bandwidth of a
link to calculate the best path to a remote network, for example, IGRP,
EIGRP, and OSPF. However, it is also important to understand that the
command has absolutely nothing to do with the speed of the
link. Yes, it would be nice to type in a command on a serial interface and
boost your bandwidth. Unfortunately, the only thing the bandwidth com-
mand is used for on an interface is to help routing protocols make smart
Delay The delay value is calculated by adding up the delay (in 10-
microsecond increments) along the path to the next router.
Reliability The reliability component of the metric is determined by how
many errors are occurring on the interface. The best possible reliability
value is 255. So, if we had an interface that was experiencing multiple
errors, and its reliability value was 128, then we would know that its reli-
ability was approximately 50 percent.
Load The load value, like the reliability value, has a maximum value of
255. However, in the case of load, lower values are better. If a particular
serial link were being used at approximately 25 percent of capacity, its
load value would be 63 (255 x .25 = 63.25). A value of 1 is the best.
MTU MTU is the Maximum Transmit Unit size, in bytes, allowed over
an interface. An Ethernet and serial interface, for example, has a default
MTU size of 1500 bytes. Traffic over an interface is more efficient at
larger MTU sizes (assuming the link is not experiencing multiple errors,
requiring retransmission), because with a larger MTU size, a message
does not have to be broken up into as many packets. Therefore, with
fewer packets, there is lower overhead (header information that is con-
tained in each packet). With lower overhead, there is a higher rate of data
An easy way to remember the metric components of IGRP is to recall the
acrostic "Big Dogs Really Like Me," where B is bandwidth, D is delay, R is reli-
ability, L is load, and M is MTU size.
Copyright ©2001 SYBEX , Inc., Alameda, CA