Operational Mechanics

Routers using a distance-vector routing protocol converge on a mutual understanding of the network's topology by sharing what they know about the network. Such routers periodically pass copies of their routing tables to their immediate network neighbors. Each recipient adds its distance-vector information to the table and forwards the modified table to its immediate neighbors. This process occurs omnidirectionally between immediately neighboring routers. Figure 10-2 uses a simple IGRP internetwork to illustrate the concept of immediate neighbors.

Figure 10-2: Each IGRP node advertises the contents of its routing table to its immediate neighbors.

Figure 10-2: Each IGRP node advertises the contents of its routing table to its immediate neighbors.

In Figure 10-2, there are four routers in an autonomous system. The gateway router is interconnected with each of the other three. It must exchange its routing information with these routers. Routers A, B, and C only have one connection each: to the gateway. Consequently, they can only exchange their information with the gateway directly. They can learn about each other's hosts through the information shared with the gateway.

Table 10-1 shows the abbreviated contents of each of the three routers' routing tables. This information is shared with the gateway router.

Table 10-1: Initial State of Routing Table Contents

Router Name

Network Address

Next Hop

Cost

A

192.168.130.0

E0

500

B

192.168.125.0

E0

500

C

192.68.253.0

T0

350

The gateway router received this information via scheduled updates from Routers A, B, and C. The gateway router uses this information to build its own routing table, the abbreviated and summarized contents of which is presented in Table 10-2. The local hosts on Router C enjoy a local cost metric from that router as they are connected via a Token-Ring network. IGRP's composite cost metric uses both Bandwidth and Delay. Therefore, it shouldn't be surprising that a deterministic 16 Mbps LAN enjoys a lower cost than a 10 Mbps contention-based LAN.

Note It is virtually impossible to provide actual valuations for the composite cost metrics in an IGRP network. This is due to the dynamic nature of some of its components, notably Load and Reliability, as well as the highly customizable constants K1 through K5. Consequently, the routing costs presented in this chapter's illustrations are fictitious. They are presented for illustrative purposes only and are not indicative of actual values that may occur.

Table 10-2: Gateway Router Routing Table Contents

Router Name

Destination

Next Hop

Number of Hops

Cost Metric

Gateway

Network 192.168.13 0

A

1

1500

Network 192.168.125

B

1

1500

Network 192.68.253

C

1

1350

Rather than maintain an entry for each host number within a given network number, routers will summarize routing information. This is done to conserve memory and CPU utilization within the router In this example, the gateway router shares the routing information in Table 10-2 with each of the other routers in the network. These routers use this information to complete their own routing tables. Table 10-3 shows the abbreviated contents of Router A's routing table after it has shared routing information with the gateway router.

Table 10-3: Router A Routing Table Contents

Router Name Destination Next Hop Number of Hops Cost Metric

A

Host 192.168.130.2

Local

0

500

Host 192.168.130.9

Local

0

500

Network 192.168.125

Gateway

2

2500

Network 192.68.253

Gateway

2

2350

Router A knows that hosts 192.168.130.2 and 192.168.130.9 are local. Datagrams addressed to these devices that are received via its serial port to the gateway router are just placed on to the Ethernet for delivery via the LAN's broadcast mechanisms. The cost metric associated with the local hosts reflects the performance of the LAN. Remember that every link has costs associated with it regardless of whether that link connects to another router!

Router A also knows that the gateway router is one hop away. Therefore, seeing that the 192.168.125.x and 192.68.253.x hosts are also one hop away from the gateway, it adds the two numbers together for a total of two hops to each machine. Each of these hops, however, has a different cost metric. This cost is IGRP's calculated composite routing metric. In a multihop route, the total cost of a route is the sum of the costs of each link in the route. Therefore, the cost of the route from 192.168.130.9 to 192.68.253.20 is 2350. This sum includes 1000 for each of the two serial links, and 350 for the LAN that the end system is connected to.

This is a highly simplified overview of the iterative process by which IGRP routers develop a consensual perspective of the network, as well as the distances between source and destination devices. If there were more than just one router interconnecting the remainder of the network's routers, convergence would require multiple iterations of this process. Needless to say, the more iterations are required to fully propagate routing information, the greater the amount of time that will elapse before all routers converge on a mutual understanding of the network's topology and distances. The network cannot be considered stable until after convergence is complete. The next section, "Topological Changes ," describes some of the ways that IGRP attempts to improve stability by minimizing convergence times.

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