Calculating Distance Vectors

Distance-vector routing protocols use metrics to keep track of the distance separating it from all known destinations. This distance information enables the router to identify the most efficient next hop to a destination that resides in a nonadjacent autonomous system.

In RFC 1058 RIP, there is a single distance-vector metric: hop count. The default hop metric in RIP is set to 1 . Therefore, for each router that receives and forwards a packet, the hop count in the RIP packet metric field is incremented by one. These distance metrics are used to construct a routing table. The routing table identifies the next hop for a packet to take to get to its destination at a minimal cost.

The earlier, proprietary RIP-like routing protocols typically used 1 as the only supported cost per hop. This convention was preserved as a default in RFC 1058 RIP, but provisions were made for the router's administrator to select higher cost values. Such values would be beneficial in discriminating between links of differing performance capabilities. These capabilities could be the bandwidths available on different network links (that is, 56 kbps versus T1 private lines) or even the performance difference between a new router versus an older model.

Typically, a cost of 1 is assigned to each of a router's ports that connect to other networks. This is apparently an artifact from RIP's pre-RFC 1058 days, when the cost per hop defaulted to 1 and was not modifiable. In a relatively small network that consisted of homogeneous transmission technologies, setting all ports to a cost of 1 would be reasonable. Figure 8-5 illustrates this.

Figure 8-5: A homogeneous network with equivalent costs.

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Figure 8-5: A homogeneous network with equivalent costs.

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A router's administrator can change the default metric. An administrator may increase the metric for slower-speed links to other routers, for example. Although this might more accurately represent the costs or distances to a given destination, this practice is not recommended. Setting the metric to a value greater than 1 makes it correspondingly easier to reach the packet's maximum hop count of 16! Figure 8-6 demonstrates how quickly routes can become invalid if route metrics are increased.

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