3.3 Route Calculation
3.3.3 Routing metrics
The routing protocol determines another important mechanism: its metric(s). There is a wide disparity in terms of both the number and the type of metrics used.

Quantity of Metrics

Simple routing protocols support as few as one or two routing metrics. More sophisticated protocols can support five or more metrics. It is safe to assume that the more metrics there are, the more varied and specific they are. Therefore, the greater the variety of available metrics, the greater your ability to tailor the network's operation to your particular needs. For example, the simple distance-vector protocols use a euphemistic metric: distance. In reality, that distance is not related at all to geographic mileage, much less to the physical cable mileage that separates source and destination machines. Instead, it usually just counts the number of router hops between those two points.

Link-state protocols may afford the capability to calculate routes based on several factors:

  • Traffic load
  • Available bandwidth
  • Propagation delay
  • The network cost of a connection (although this metric tends to be more of an estimate than an actual value)

Most of these factors are highly dynamic in a network; they vary by time of day, day of week, and so forth. The important thing to remember is that as they vary, so does the network's performance. Therefore, the intent of dynamic routing metrics is to allow optimal routing decisions to be made using the most current information available.

Static Versus Dynamic Metrics

Some metrics are simplistic and static, whereas others are highly sophisticated and dynamic. Static metrics usually offer the capability to customize their values when they are configured. After this is done, each value remains a constant until it is manually changed.

Dynamic protocols enable routing decisions to be made based on real-time information about the state of the network. These protocols are supported only by the more sophisticated link-state or hybridized routing protocols.