Foundations of Routing Protocols
Dynamic routers use routing protocols to facilitate the ongoing communication and dynamic updating of routing tables. Routing protocols are used between routers and represent additional network traffic overhead on the network. This additional traffic can become an important factor in planning WAN link usage. RIP and OSPF for IP, and RIP and NLSP for IPX are all routing protocols. In some cases, such as RIP for IP (version 1) and RIP for IPX, the routing information is exchanged using MAC-level broadcasts.
An important element of a routing protocol implementation is its ability to sense and recover from internetwork faults. How quickly it can recover is determined by the type of fault, how it is sensed, and how the routing information is propagated through the internetwork.
When all the routers on the internetwork have the correct routing information in their routing tables, the internetwork has converged. When convergence is achieved, the internetwork is in a stable state and all routing occurs along optimal paths.
When a link or router fails, the internetwork must reconfigure itself to reflect the new topology. Information in routing tables must be updated. Until the internetwork reconverges, it is in an unstable state in which routing loops and black holes can occur. The time it takes for the internetwork to reconverge is known as the convergence time. The convergence time varies based on the routing protocol and the type of failure (downed link or downed router).
Routing protocols are based either on a distance vector or link state technology. The main differences between distance vector and link state routing protocols include the following:
What routing information is exchanged.
How the information is exchanged.
How quickly the internetwork can recover from a downed link or a downed router.