Distance vector routing algorithm is a class of routing algorithms calling for each router to send some or its entire routing table to its neighbors.
Distance Vector protocols determine the best path based on how far the destination is, while Link State protocols are capable of using more sophisticated methods considering link variables, such as bandwidth, delay, reliability and load (Osterloh 2002). Distance can be hop-count or a combination of metrics calculated to represent a distance value.
Distance vector routing protocols keep track of changes to the internetwork by broadcasting periodic routing updates out all active interfaces. Each broadcast includes the complete routing table. This can work okay, but the amount of CPU process and link bandwidth can be more than you might want. Plus, the slow convergence of distance vector routing protocols can result in inconsistent routing tables and routing loops (Patrick J. Conlan 2009).
To minimize the impact of routing loops, routing protocols can use one or more loop avoidance mechanisms. The different techniques are (Osterloh 2002):
 |
Count to infinity: sets a maximum hop count value, that, when exceeded, equates to infinity hops (or |
|
destination unreachable). |
|
Holddowns: start as soon as a router receives an update from a neighbor indicating that an attached network |
|
has gone down. Until the timer elapses, the router ignores updates regarding this route from other routers unless it receives an update from the neighboring router that initially informed the network of the downed link. The timer stops if it receives a message from the neighboring router. At that point, the network is marked as reachable again and the route table is updated. |
|
Split horizon: prevents information being sent back in the direction from which that information was received. |