Routing updates can occur using the distance vector approach or the link-state approach.
• Distance-vector protocols use a routine, periodic announcement that contains the entire contents of the
routing table. These announcements are usually broadcasts and are propagated only to directly-connected,
next-hop, devices. This allows the router to view the network from the neighbor's perspective and
facilitates the addition of the router's metric to the 'distance' already stated by the neighboring router.
However, this approach uses considerable bandwidth at regular intervals on each link even if no
topology changes have occurred.
• Link-state protocols use a triggered-update type of announcement that is generated only when there is a
topology change within the network. The link-state announcements only contain information about the
link that changed and are propagated or flooded to all devices in the network. This approach saves
bandwidth on each link because the announcements contain less information and is only sent when there
is a topology change. In some link-state protocols, a periodic announcement is required to ensure that the
topology database is synchronized among all routing devices.