Some Background on Router Protocols
A protocol is a formalized system for
exchanging a specific type of information in a certain way. A
routing protocol formalizes the
continually ongoing exchange of route information between routers.
Messages called routing updates pass
information used by routers to determine and calculate paths to
destinations. Determining and calculating these paths is done by
routing algorithms which are composed
of a system of rules that control a network's behavior so that routers can
adapt to changing circumstances in the network's topology. Two
examples of these ongoing changes can be the implementation of some new
equipment on the network or temporarily out of service routers on the
network. A routing table is a
list of routes available to forward traffic to various destinations.
Each router typically maintains a single routing table, not one for each
Routing protocols are set up to interact in a peer to peer arrangement where each router plays a relatively equal role. They are definitely not coordinated by some unique central administration across the internet. The routing algorithms tend to share the following two basic approaches.
Topology Changes. How do routers sense topology changes? There are a fairly constant stream of messages sent at different time intervals so that routers can inform their neighbors about their status. The following bulleted list represents how routers "sense" down links.
But sensing a topology change is only the first step. As soon as a change is sensed, routing updates must be passed until all routers can converge on a new topology by incorporating the change. Sometimes these updates can cause misinformation to be propagated through the network. Due to propagation delays, not all routers receive updates at the same moments. The impact of these propagation delays on convergence after topology changes increases as the size of the internetworks increase.
These propagation delays would not likely cause any problems to routing protocols if routers always converged before any new changes occurred. Unfortunately, this is not always the case. A routing loop occurs when payload packets can't reach their destinations because of conflicting routing table information. This happens in large internetworks when subsequent topology changes occur before routers have converged due to previous topology changes.
Routing protocols incorporate a number of sophisticated mechanisms to thwart the onset of routing loops.
Routing Protocols. A routing metric is a value used by a routing protocol to influence routing decisions. The mostly widely used metrics follow along with some discussion.
Implementing these there are three basic types of routing protocols along with some of their best known implementations.
Finally, it is always important to distinguish whether a router is inside, outside or on an interactive boundary or at the gateway of a network.