Networking Concepts Review - CCIE Routing and Switching Exam Certification Guide - page 40

four T1 links, the first path is selected because of the lower number of hops (see Figure 2-8).
However, this is not necessarily the fastest path. If you have a 20 MB file, you want to transfer
it through T1 links rather than 56 k links.

Hop Count Metric Prefers Fewer Hops Figure 2-8

3 hop counts

56 kbps 56 kbps

T1

T1

T1

T1

5 hop counts

Bandwidth
The bandwidth metric uses the default or configured interface bandwidth. With this metric, Fast
Ethernet (100 Mbps) is preferred over DS-3 (45 Mbps). You can alter the metric by using the
interface command bandwidth speed , where the speed is entered in kbps. To configure a serial
interface for 128 kbps, use the following commands:

router3(config)#interface serial 0
router3(config-if)#bandwidth 128
router3(config-if)#
Use the show interface command to verify the configured bandwidth:

router3>show interface serial 0
Serial0 is up, line protocol is up
Hardware is PQUICC Serial
MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec, rely 254/255, load 1/255
Encapsulation HDLC, crc 16, loopback not set, keepalive set (10 sec)

BW 128 Kbit

Cost
OSPF and IS-IS use the cost metric, which is can be derived from the bandwidth of the
interface. IS-IS assigns a default cost of 10 for all interfaces. It does not use the auto cost
determination that OSPF uses.

To calculate cost in OSPF, use the following formula:
10 8 / BW, where BW is the default or configured bandwidth of the interface.