Diffusing Update Algorithm, or DUAL, is an enhanced distance vector algorithm. EIGRP uses it to calculate the shortest path to a destination within a network.
Whilst EIGRP acts in a similar way to advertising its networks as other distance vector protocols do, there are some enhancements:
- Rapid convergence time when the network topology changes
- Updates are only sent when there is a change in the network. Full updates are not sent periodically as other distance vector protocols do.
- Hellos and neighbour relationships are formed in the same way as link state protocols.
- Factors other than hop count are taken into consideration, such as
- Maximum Transmission Unit (MTU) setting
- There is an option to load balance across equal or unequal cost paths.
With this additional link-state style features on top of being a distance vector protocol, EIGRP can be referred to as a hybrid routing protocol since it can refer on other vectors that just simply hop-count.
On EIGRPs default setting, EIGRP just advertises the total path delay and minimum bandwidth for a route to every neighbour. Each neighbour can perform calculations to determine the best path based on information collected from its directly attached neighbours.