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.