A dynamic routing protocol must include a set of procedures for a router to inform other routers about its
directly connected networks, to receive and process the same information from other routers, and to pass
along the information it receives from other routers. Further, a routing protocol must define a metric by
which best paths may be determined.
A further criteria for routing protocols is that the reachability information in the route tables of all routers
in the internetwork must be consistent. If router A in Figure 4.1 determines that the best path to network
192.168.5.0 is via router C and if router C determines that the best path to the same network is through
router A, router A will send packets destined for 192.168.5.0 to C, C will send them back to A, A will
again send them to C, and so on. This continuous circling of traffic between two or more destinations is
referred to as a routing loop.
NOTE
Convergence and convergence time
The process of bringing all route tables to a state of consistency is called convergence. The time it takes
to share information across an internetwork and for all routers to calculate best paths is the convergence
time.
Figure 4.2 shows an internetwork that was converged, but now a topology change has occurred. The link
between the two left-most routers has failed; both routers, being directly connected, know about the
failure from the data link protocol and proceed to inform their neighbors of the unavailable link. The
neighbors update their route tables accordingly and inform their neighbors, and the process continues
until all routers know about the change.
Figure 4.2. Reconvergence after a topology change takes time. While the internetwork is in an unconverged
state, routers are susceptible to bad routing information.
Notice that at time t2 the three left-most routers know about the topology change but the three right-most
routers have not yet heard the news. Those three have old information and will continue to switch packets
accordingly. It is during this intermediate time, when the internetwork is in an unconverged state, that
routing errors may occur. Therefore convergence time is an important factor in any routing protocol. The
faster a network can reconverge after a topology change, the better.