Classification of Dynamic Routing Protocols

Dynamic routing protocols are based on an algorithm, such as Bellman-Ford-Fulkerson, Dijkstra SPF (Shortest Path First), or the Enhanced Interior Gateway Routing Protocol (EIGRP) DUAL (Diffuse Update Algorithm). Based on these algorithms, dynamic IGPs can be classified in link-state and distance-vector protocols.

NOTE

The Border Gateway Protocol (BGP) discussed in the next chapter represents a path-vector protocol essentially based on a distance-vector approach as well.



The main task of these protocols is path determination and calculation. With multiple paths to a destination prefix, the protocol makes intrinsic decisions based on metrics/cost/preference assigned to routes. Such a label is a measure of preference within a particular routing protocol. It can be simple, such as hop count for RIP, or a composite metric such as with EIGRP based on load, reliability, delay, and bandwidth, or cost based in a generic way such as with OSPF.

Link-State Protocols
Link-state protocols such as OSPF are cost-based, and the cost is usually derived from the link bandwidth. When a protocol has a stable view of the topology, it is referred to as having converged or achieved equilibrium. Do not confuse this view with the notion of converged networks meaning voice, video, data, and storage over one consolidated IP infrastructure.

The task of computing shortest paths in a network is a mathematical problem tackled with graph theory. You will read more about that in the section "Introduction to Link-State Routing Protocols" later in this chapter. Nevertheless, one cannot argue that link-state protocols are superior in every aspect per se.

Distance-Vector Protocols
Distance-vector protocols usually broadcast full table updates. Deviation from this case is referred to as an asynchronous, triggered, flash, or incremental update.

Note the following:

The name distance vector is derived from the fact that routes are advertised as vectors of (distance, direction), where distance is defined in terms of a metric and direction is defined in terms of the next-hop router.[1]

For loop prevention, simple split horizon or split horizon with poisoned reverse is used in distance-vector protocols. A thorough discussion of loop detection, prevention, and termination goes beyond the scope of this book. I recommend Jeff Doyle's two volumes of Routing TCP/IP (Cisco Press; 1998 and 2001, respectively) for further information.