Multiple entries up to a maximum of 104

Version will always be set to one.

Opcode will be one for an IGRP Request packet and two for an IGRP Update packet. A Request packet consists of a header with no entries.

Edition is incremented by the sender of an update whenever there is a change of routing information. The edition number helps the router avoid accepting an old update that arrives after the newer update.

Autonomous System Number is, more accurately, the ID number of the IGRP process. This tag allows multiple IGRP processes to exchange information over a common data link.

Number of Interior Routes is the number of entries in the update that are subnets of a directly connected network. If none of the directly connected networks is subnetted, this field will be zero. Interior route entries always appear first in the update. This field, along with the subsequent fields for number of system routes and number of exterior routes, tell an IGRP process how many 14-octet entries are contained in the packet and hence the packet length.

Number of System Routes tells the number of routes to networks that are not directly connected—in other words, routes that have been summarized by a network border router. The entries numbered in this field, if any, follow the interior route entries.

Number of Exterior Routes is the number of routes to networks that have been identified as default networks. The entries numbered in this field, if any, appear last in the update.

Checksum is calculated on the IGRP header and all entries. To calculate the checksum, the field is set to zero and the 16-bit one's complement sum of the packet (not including the IP header) is calculated. The 16-bit one's complement of that sum is then stored in the checksum field. Upon receipt, the 16-bit one's complement of the packet is again calculated, this time including the transmitted checksum field. When performed on an error-free packet, the result will be all ones (OxFFFF).

Destination is the first field of each route entry. It may seem odd at first glance that the field is only three octets long, given that IP addresses are four octets. As it turns out, a destination can be made recognizable in three octets because of IGRP's route categorization. If the entry is an interior route, at least the first octet of the IP address will always be known from the address of the interface on which the update was received. Therefore, the destination fields of interior route entries will contain only the last three octets of the address. Similarly, if the entry is a system or external route, the route will have been summarized and at least the last octet will be all zeros. Therefore the destination fields of system and external route entries will contain only the first three octets of the address.

For example, if an interior route of 20.40.0 is received on interface 172.20.1.1/24, it is recognized as subnet 172.20.40.0/24. Similarly, if system routes 192.168.14 and 20.0.0 are received, IGRP will understand these destinations as major network addresses 192.168.14.0 and 20.0.0.0.

Delay is the 24-bit DLYIGRP(sum) previously explained—the sum of the configured delays expressed in units of 10 microseconds.

Bandwidth is the 24-bit BWIGRP(min) previously explained—10,000,000 divided by the lowest configured bandwidth of any interface along the route.

MTU is the smallest Maximum Transmission Unit of any link along the route to the destination. Although an included parameter, it has never been used in the calculation of metrics.

Reliability is a number between 0x01 and 0xFF that reflects the total outgoing error rates of the interfaces along the route, calculated on a five-minute exponentially weighted average.

Load is also a number between 0x01 and 0xFF, reflecting the total outgoing load of the interfaces along the route, calculated on a five-minute exponentially weighted average.

Hop Count is a number between 0x01 and 0xFF indicating the number of hops to the destination. A router will advertise a directly connected network with a hop count of 0; subsequent routers will record and advertise the route relative to the next-hop router. For example, in Figure 6.8 Casablanca shows subnet 172.20.40.0 to be two hops away. Figure 6.7 shows the meaning of the hop count: 172.20.40.0 is two hops from the next-hop router, Teheran.

An analyzer decode of a portion of an IGRP update packet is shown in Figure 6.10.

Figure 6.10. The header and the first entry of an IGRP update can be seen in this analyzer decode.

Figure 6.10. The header and the first entry of an IGRP update can be seen in this analyzer decode.

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