Distance Vector Routing Protocol features and loop prevention

Distance vector describes what a router knows about each route.  At the end of the process, when a router learns about a route to a subnet, all the router knows is some measurement of distance (the metric) and the next-hop router and outgoing interface to use for that route (a vector, or direction).

Distance Vector Operation in a Stable Network

Distance vector routing protocols send periodic full routing updates.

* Periodic: The hourglass icon represents the fact that the updates repeat on a regular cycle.  RIP uses a 30-second update interval by default.

* Full updates: The routers send full updates every time instead of just sending new or changed routing information.

* Full updates limited by split-horizon rules: The routing protocol omits some routes from the periodic full updates because of split-horizon rules.  Split horizon is a loop-avoidance feature that is covered in the next few pages.

Distance Vector Loop Prevention

Routing loops occur when the routers forward packets such that the same single packet ends up back at the same routers repeatedly, wasting bandwidth and never delivering the packet.