Cisco Systems CCNA 2 9.1.6 Determining the route metric , 9.1.7 Determining the route next hop , 9.1.8 Determining the last routing update

Explain administrative distances to the students. The administrative distance represents the

trustworthiness of the source of a route. The Cisco IOS is designed to trust directly connected

routes more than any other source. Directly connected routes have the lowest administrative

distance of zero. The IOS also trusts routes that are configured by a network administrator,

which are static routes. These have an administrative distance of one. Students should also

learn the administrative distances of RIP, IGRP, EIGRP, and OSPF. The administrative

distance must not be confused with routing metrics. Metrics are calculated and compared

among routes from the most trusted routing source. The router will select the route from the

best administrative source with the lowest metric. This is an important concept for the students

to understand.

9.1.6 Determining the route metric

Routing protocols use metrics to determine the best route to a destination. The metric is a

value that measures the desirability of a route. Some routing protocols use only one factor to

calculate a metric. For example, RIP only uses hop count as a metric. Other protocols base

their metric on bandwidth, delay, load, reliability, ticks, maximum transmission unit, and cost.

Discuss with the students the differences between each of these metrics so they fully

understand what is used to calculate the best route.

Each routing algorithm interprets what is best in its own way. The algorithm generates a

number, called the metric value, for each path through the network. The smaller the metric

value is, the better the path. Review the administrative distances covered in an earlier section.

Make sure students understand the difference between administrative distances and metrics.

Also explain that routes from different protocols cannot be compared since routing protocols

use different metrics and different methods to determine the metric value.

9.1.7 Determining the route next hop

Routing algorithms fill routing tables with information. Destination or next hop associations tell

a router that a particular destination can be reached if the packet is sent to a particular router

that represents the next hop on the way to the final destination. Have students look at routing

table examples and determine the next-hop router for a network.

9.1.8 Determining the last routing update

A network administrator can use the following commands to find the last route update:

• show ip route

• show ip route network

• show ip protocols

• show ip rip database

Stress the importance of these commands. Use examples to show the students the

information generated by these commands.