In order for routing to work correctly, each router must ensure that its neighbor routers are aware of the status of all networks in the autonomous system. This is a process called "convergence," and when all routers in an autonomous system are aware of the status of all networks, the network is said to have "converged." On the Washington Elementary School District network, routers will be using IGRP as their routing protocol to exchange this information.
When the router is first powered on and loads its configuration, it first establishes which networks are directly connected to it. In the case of the Desert View router, the three directly attached networks are 192.168.6.0, 192.168.7.0, and 168.192.8.0. The router then begins building its routing table. The routing table consists of destination network address and next hop pairs, which tell the router the interface to use for forwarding packets to a particular network, and a metric.
The metric represents the "value" or "cost" of the path, and in case multiple paths to the same destination network exist, the router compares the metric on each path to determine the best route. Different routing protocols use different metrics to determine the best route. IGRP uses bandwidth and delay by default, and can also use reliability and load.
The routers then begin sending copies of their routing tables to their neighbor routers.
As routers receive updates, they gradually build a detailed picture of the entire network topology, allowing each router to determine the best path to a network destination.
With the router command sequence listed above, the router will send a full copy of its routing table to its neighbor router or routers every 90 seconds. IGRP also uses "flash updates" to send an update sooner than 90 seconds if a change occurs, such as a network going down. If the router does not receive an update regarding a route within 3 update periods (i.e. 270 seconds in the command sequence above) it will declare the route invalid or inaccessible. If the router does not receive an update regarding a route within 7 update periods (i.e. 630 seconds in the command sequence above) it will flush the route from its routing table entirely.
It is desirable that the routers in the autonomous system converge quickly, because inconsistent routing information between routers can cause routing loops or network outages. When all the routers share the same routing information, then information can be transmitted from source to destination quickly and efficiently. The more frequently routing updates occur, the faster the network will converge – however, this will use bandwidth on the link, causing utilization to decline. One must balance the stability of the network and the frequency of network changes against the need to preserve bandwidth for user traffic rather than routing table updates. The Desert View router should not need to send updates very frequently, as the connection to the administrative and curriculum LANs is fairly stable and not very likely to go down. Therefore, in the command sequence listed above, routing updates occur every 90 seconds, which is the default for IGRP, and preserving bandwidth on the T1 link to the Phoenix N.W. office.
