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Cisco Systems developed IGRP to overcome the limitations of RIPv1. IGRP is a distance-vector routing protocol that considers a composite metric that, by default, uses bandwidth and delay as parameters instead of hop count. IGRP is not limited to RIP's 15-hop limit. IGRP has a maximum hop limit of 100 by default and can be configured to support a network diameter of 255.

Note

IGRP is no longer a CCDA test topic. EIGRP is the enhanced version of IGRP. However, reading this section will provide a good foundation for learning EIGRP in the section that follows.

With IGRP, routers usually select paths with a larger minimum-link bandwidth over paths with a smaller hop count. Links do not have a hop count. They are exactly one hop.

IGRP is a classful protocol and cannot implement VLSM or CIDR. IGRP summarizes at network boundaries. As in RIP, IGRP implements split horizon with poison reverse, triggered updates, and holddown timers for stability and loop prevention. Another benefit of IGRP is that it can load-balance over unequal-cost links. As a routing protocol developed by Cisco, IGRP is available only on Cisco routers.

By default, IGRP load-balances traffic if several paths have equal cost to the destination. IGRP does unequal-cost load balancing if configured with the variance command.

IGRP Timers

IGRP sends its routing table to its neighbors every 90 seconds. IGRP's default update period of 90 seconds is a benefit compared to RIP, which can consume excessive bandwidth when sending updates every 30 seconds. IGRP uses an invalid timer to mark a route as invalid after 270 seconds (3 times the update timer). As with RIP, IGRP uses a flush timer to remove a route from the routing table; the default flush timer is set to 630 seconds (7 times the update period and more than 10 minutes).

If a network goes down or the metric for the network increases, the route is placed in holddown. The router accepts no new changes for the route until the holddown timer expires. This setup prevents routing loops in the network. The default holddown timer is 280 seconds (3 times the update timer plus 10 seconds). Table 10-2 summarizes the default settings for IGRP timers.

Table 10-2. IGRP Timers

IGRP Timer	Default Time
Update	90 seconds
Invalid	270 seconds
Holddown	280 seconds
Flush	630 seconds

IGRP Metrics

IGRP uses a composite metric based on bandwidth, delay, load, and reliability. Chapter 9 discusses these metrics. By default, IGRP uses bandwidth and delay to calculate the composite metric, as follows:

IGRP_metric = {k1 * BW + [(k2 * BW)/(256 – load)] + k3 * delay} * {k5/(reliability + k4)}

In this formula, BW is the lowest interface bandwidth in the path, and delay is the sum of all outbound interface delays in the path. The router dynamically measures reliability and load. The values of reliability and load used in the metric computation range from 1 to 255. Cisco IOS routers display 100 percent reliability as 255/255. They also display load as a fraction of 255. They display an interface with no load as 1/255. By default, k1 and k3 are set to 1, and k2, k4, and k5 are set to 0. With the default values, the metric becomes

IGRP_metric = {1 * BW + [(0 * BW)/(256 – load)] + 1 * delay} * {0/(reliability + 0)}

IGRP_metric = BW + delay

The BW is 10,000,000 divided by the smallest of all the bandwidths (in kbps) from outgoing interfaces to the destination. To find delay, add all the delays (in microseconds) from the outgoing interfaces to the destination, and divide this number by 10. (The delay is in 10s of microseconds.)

Example 10-3 shows the output interfaces of two routers. For a source host to reach network 172.16.2.0, a path takes the serial link and then the Ethernet interface. The bandwidths are 10,000 and 1544; the slowest bandwidth is 1544. The sum of delays is 20000 + 1000 = 21000.

Example 10-3. show interface

RouterA> show interface serial 0 Serial0 is up, line protocol is up Hardware is HD64570 Internet address is 172.16.4.1/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 RouterB> show interface ethernet 0 Ethernet0 is up, line protocol is up Hardware is Lance, address is 0010.7b80.bad5 (bia 0010.7b80.bad5) Internet address is 172.16.2.1/24 MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, reliability 255/255, txload 1/255, rxload 1/255

The IGRP metric is calculated as follows:

IGRP_metric = (10,000,000/1544) + (20000 + 1000)/10

IGRP_metric = 6476 + 2100 = 8576

You can change the default metrics using the metric weight tos k1 k2 k3 k4 k5 subcommand under router igrp. Cisco once intended to implement the tos field as a specialized service in IGRP. However, it was never implemented, so the value of tos is always 0. The k arguments are the k values used to build the composite metric. For example, if you want to use all metrics, the command is as follows:

router igrp n
 metric weight 0 1 1 1 1 1

IGRP Design

IGRP should not be used in the design of new networks because it does not support VLSMs. The IP addressing scheme with IGRP requires the same subnet mask for the entire IP network, a flat IP network. IGRP does not support CIDR and network summarization within the major network boundary. IGRP is not limited to a maximum of 15 hops as RIP is; therefore, the network diameter can be larger than that of networks using RIP. IGRP also broadcasts its routing table every 90 seconds, which produces less network overhead than RIP. IGRP is limited to Cisco-only networks.

Drawbacks of IGRP are that it lacks VLSM support and that it broadcasts its entire table every 90 seconds. Its slow convergence makes it too slow for time-sensitive applications. EIGRP is recommended over IGRP.

As shown in Figure 10-7, when you use IGRP, all segments must have the same subnet mask.

IGRP Summary

The characteristics of IGRP follow:

• Distance-vector protocol.
• Uses IP protocol number 9.
• Classful protocol (no support for CIDR).
• No support for VLSMs.
• Composite metric using bandwidth and delay by default.
• You can include load and reliability in the metric.
• Route updates are sent every 90 seconds.
• 104 routes per IGRP message.
• Hop count is limited to 100 by default and is configurable up to 255.
• No support for authentication.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• By default, equal-cost load balancing. Unequal-cost load balancing with the variance command.
• Administrative distance is 100.
• Previously used in large networks; now replaced by EIGRP.