"Metrics" in Routing Protocols

A Brief Explanation

Metrics

In virtually every routing protocol, there is the concept of "metric." The word means "measure," and readers familiar with the International System of Units, the "metric system," will realize that this is actually a system of various measurements, from mass to length to time, etc., etc.

In the realm of routing protocols, "metric" also means a measure; in this case, it is a measure of how good/desirable or how bad/undesirable it is to actually use a particular route. Different protocols can have very different ideas of what constitutes a metric; both

• the range of valid values of "metric" and
• the direction of increasingly better metrics can vary (i.e., whether a "low" metric number, say, near zero, is good or bad).

This means that "metric" in the sense generally used, since it means a measure as defined within a given routing protocol, is a valid comparison only for evaluating routes within the confines of that routing protocol. In general, for this meaning of the term "metric," the comparison of metrics associated with routes obtained from different routing protocols is not valid. In fact, such direct comparison is both fundamentally flawed and a dangerous practice.