In a routed network, the routing process in each router must maintain a loop-free, single path to each
possible destination logical network. When all of the routing tables are synchronized and each contains a
usable route to each destination network, the network is described as being 'converged'. Convergence is the
time it takes for all routers to agree on the network topology after a change in the network.
Convergence efforts are different within different routing protocols. There are at least two different
detection methods used by all routing protocols. The first method is used by the Physical Layer (Layer 1)
and the Data Link Layer (Layer2) protocols. When the network interface on the router does not receive three
consecutive keepalives, the link will be considered down. The second method is that when the routing
protocol at the Network/Transport Layer (Layer 3) fails to receive three consecutive Hello messages, the
link will be considered down.
Routing protocols have timers that are used to stop network loops from occurring on a network when a link
failure has been detected. Hold-down timers are used to give the network stability while new route
calculations are being performed. They also allow all the routers a chance to learn about the failed route to
avoid routing loops and counting to infinity problems. Since a network cannot converge during this holddown
period, this can cause a delay in the routing process of the network. Because of this slow convergence
time, link-state routing protocols do not use hold-down timers.