The first of these categories is distance-vector protocols. In a distance-vector protocol, a router periodically sends all of its neighbor’s two pieces of information about the destinations it knows how to reach. First, the router tells its neighbors how far away it thinks the destination is; second, it tells its neighbors what direction (or vector) to use to get to the destination.
This direction indicates the next hop that a listener should use to reach the destination, and typically takes the form "send it to me, I know how to get there." For example, RIP route updates simply list a set of destinations that the announcing router knows how to reach, and how far away it thinks each destination is. The receiver infers that the next hop to use is the announcing router. However, an update can also take the form "send it to this other router who knows how to get there." This second form is usually used only when the router that should be used to reach the destination cannot (or will not) speak the routing protocol being used by the other routers. Not all routing protocols support this form of third-party route update.
The other part of the protocol, the distance, is where distance-vector protocols differ. In each case, the protocol uses some metric to tell the receiving routers how far away the destination is. This metric may be a true attempt at measuring distance (perhaps using a periodic measure of the round trip time to the destination), something that approximates distance (such as hop count), or it may not measure distance at all.
Instead, it may attempt to measure the cost of the path to the destination. It may even involve a complex computation that takes into account factors like network load, link bandwidth, link delay, or any other measure of the desirability of a route. Finally, it may include an administrative weight that is set by a network administrator to try to cause one path to be preferred over another.
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