What Is a Routing Protocol?

A routing protocol can be analyzed as a process. A protocol is formalized through the Request for Comments (RFC) process. This process involves open written commentary on a proposed technology in an effort to bring about standardization of that technology. An example of a routing protocol standard would be OSPF as defined in RFC 2328.

The basic reason you are studying this topic is to understand how, through the use of OSPF, you can get packets from one host to another over a network. This process can be summarized, from a router's perspective, as consisting of two steps, forwarding and routing:

• Forwarding-Refers to the process of a router receiving a packet on an interface and then knowing which interface to retransmit that packet out of so that the packet can continue to its destination. However, forwarding is entirely dependent on the router knowing where to send the packet; if it does not know this, it just discards the packet.
• Routing-For the router to know in which direction to forward packets, it requires a route or road map that illustrates the path from source to destination. This route can be generated by either a static or dynamic configuration; at this point, you need to be concerned with only the dynamic aspect of routing. OSPF is a dynamic routing protocol, and when a router is running OSPF, it dynamically develops routes to all destinations within a network. You can view these routes when you view a router's routing table.

The following are broad categories of protocols:

• Routed protocols-Protocols that forward data via routers. A router must be able to interpret the logical network as specified by the routed protocol for the router to operate properly. The most commonly known routed protocol is Internet protocol (IP); other examples include AppleTalk and DECnet. Routed protocols rely on the routing protocols for transport over a LAN or WAN.
• Dynamic routing protocols-Protocols that accomplish dynamic routing with a routing algorithm. A dynamic routing protocol supports a routed protocol and maintains routing tables. A dynamic routing protocol dynamically exchanges information about paths or topology of the network by distributing routing information throughout a network. Examples of dynamic routing protocols include OSPF, interior gateway routing protocol (IGRP), and routing information protocol (RIP).

A routed protocol such as IP is used as the method of communication between devices on a network. Using a selected routing protocol, such as OSPF, which is supported by the routed protocol, such as IP, you can build the network so that every device can communicate. For example, as previously mentioned, IP is a routed protocol that can use either OSPF or RIP as its routing protocol.

In summary, a dynamic routing protocol is a set of standardized rules that allow routers to determine routes. The routing protocol builds routing tables that tell the router the optimal path to a destination. Routing protocols compare numeric values known as metrics to determine the optimal route (sometimes referred to as path). Metrics are numeric values that represent route path characteristics. Metrics can be thought of as costs; therefore, the numeric value is explained as the cost of transiting a link. This information is then stored in the routing table to be used by the router when determining the best route to a destination network.