Convergence

Convergence
Convergence time is the time it takes for all routers to agree on the network topology after a
change in the network. The routers have synchronized their routing tables.
There are at least two different detection methods used by all routing protocols. The first
method is used by the Physical and Data Link layer protocols. When the network interface on
the router does not receive three consecutive keepalives, the link will be considered down.
The second detection method is that when the routing protocol at the Network and Transport
layers fails to receive three consecutive Hello messages, the link will be considered down.
After the link is considered down is when the routing protocols differ. Routing protocols have
timers that are used to stop network loops from occurring on a network when a link failure has been
detected. Holddown 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. Because a routing domain cannot converge during
this holddown period, this can cause a delay in the routing process of the domain. Because of this
slow convergence penalty, link-state routing protocols do not use holddown timers.
The following section describes the convergence process for RIP, IGRP, EIGRP, and link-state
protocols when a link failure occurs in a network.