IP Routing

Convergence

Changes -- planned and unplanned -- are normal in any network:

• A serial link breaks

• A new serial link is added to a network

• A router or hub loses power or malfunctions

• A new LAN segment is added to a network

All routers in the routing domain will not reflect these changes right away. This is because RIP routers rely on their direct neighbors for routing updates, which in turn rely on another set of neighbors. The routing process that is set into motion from the time of a network change (such as the failure of a link) until all routers correctly reflect the change is referred to as convergence. During convergence, routing connectivity between some parts of the network may be lost and, hence, an important question that is frequently asked is “How long will the network take to converge after such-and-such failure in the network?” The answer depends on a number of factors, including the network topology and the timers that have been defined for the routing protocol.

The following list defines the four timers that are key to the operation of any DV protocol, including RIP:

Update timer (default value: 30 seconds)

After sending a routing update, RIP sets the update timer to 0. When the timer expires, RIP issues another routing update. Thus, RIP updates are sent every 30 seconds.

Invalid timer (default value: 180 seconds)

Every time a router receives an update for a route, it sets the invalid timer to 0. The expiration of the invalid timer indicates that six consecutive updates were ...