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166 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

Each route lists the subnet number, the outgoing interface, and possibly the next-hop router’s IP address. The next-hop router IP address is preceded by the word “via” in the output. Directly connected routes do not have a next-hop router field, because the packet does not need to be sent to another router, but instead to the destination host.

Finally, the numbers between the brackets mention some very useful information. The first number represents the administrative distance, which is covered later in this chapter. The second number lists the metric associated with this route. For instance, in the route to subnet, the administrative distance is 100, and the metric is 8539. Remember, IGRP calculates the metric as a function of bandwidth and delay, so the actual metric value typically is a much larger number than the hop count metric shown in the examples earlier in this chapter.

IGRP Metrics

IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability. By default, only bandwidth and delay are considered; the other parameters are considered only if they are enabled via configuration. Delay and bandwidth are not measured values but are set via the delay and bandwidth interface subcommands. (The same formula is used to calculate the metric for EIGRP, but with a scaling factor so that the actual metric values are larger, allowing more granularity in the metric.)

The show ip route command in Example 5-6 shows the IGRP metric values in brackets. For example, the route to shows the value [100/8539] beside the subnet number. Administrative distance is covered briefly near the end of this chapter. The metric 8539 is a single value, as calculated based on bandwidth and delay. The metric is calculated (by default) as the inverse of the minimum bandwidth, plus the cumulative delay on all links in the route. In other words, the higher the bandwidth, the lower the metric; the lower the cumulative delay, the lower the metric.

IGRP uses the value set with the bandwidth command on each interface to determine the interface’s bandwidth. On LAN interfaces, the bandwidth command’s default values reflect the correct bandwidth. However, on serial interfaces, the bandwidth command defaults to 1544—in other words, T1 speed. (The bandwidth command uses units of kbps, so the bandwidth 1544 command sets the bandwidth to 1544 kbps, or 1.544 Mbps.)

The router cannot figure out the actual physical clock rate and automatically change the bandwidth command value for a serial interface. For IGRP to choose the best route, the bandwidth should be configured to the correct value on each interface. For example, Figure 5-10 shows three examples of the same network, with different results for R1’s routes based on the routing protocol and the bandwidth settings.