Internetworking Protocols

Data and information exchange is what internetworking (including the Internet) is all about. In order for end-systems to communicate with one another across often incompatible networks (both physically and logically) common ground had to be found. This need for some form of common ground in order to facilitate the exchange of information between end-points is the reason that routing; the process by which internetwork information exchanges are accomplished, was developed.

Router

Router is the most wide-spread and commonly adopted name used when referring to devices that perform traffic flow, traffic regulation, network accessibility, internetworking and other routing functions. That is to say routers facilitate/perform data transfers between different networks as well as across network boundaries.

In order to do its job a router needs to know via which interface any given network can be reached. It is also desirable for them to know; in advance, of alternative routes usable in the event of the “primary” route becoming unavailable.

Three Classes of Internetworking Protocols

From the perspective of an internetworking device; such as a router, there are only three classes of networking protocols: Routing Protocols, Routable Protocols and Non-Routable Protocols.

Routing Protocols

It is specialty software known as routing protocols that sends/receives routing information packets to/from other routers. This allows the router to:

• Dynamically Learn and Advertise Routes
• Determine Possible Routes, Route Availability, Route Efficiency, Best Route to Destination and Alternative Routes

Routing Protocols Include:

• Routing Information Protocol (RIP)
• Routing Information Protocol II (RIP II)
• Open Shortest Path First (OSPF)
• Intermediate System to Intermediate System (IS-IS)
• Interior Gateway Routing Protocol (IGRP)
• Enhanced Interior Gateway Routing Protocol (EIGRP) (Cisco proprietary protocol)
• Border Gateway Protocol (BGP)

Note that because Interior Gateway Protocols (IGPs) are used to communicate routing information between routers within an autonomous system they are routing protocols and not routed protocols as many people mistakenly believe.

Dynamic Route Advertisement - The most common mechanism used to for router-to-router communication and routing information interchanges is through the use of advertisements. To illustrate this here is a simple version of what routing protocols get up to:

Router1: “Hey Neighbour! Check this out. It’s really good and I know you’ll love it because it is my routing table. So check it out to see if there is anything in it that you don’t know already.”

Router2: “Well thanks a lot; much appreciated. Here is my routing table so please do likewise. Be talking again with you real soon.”

Routable Protocols

Routable protocols on the other hand are protocols that are capable of being forwarded from one router to another (routed). Routable protocols contain the data elements (IP Addresses etc) so essential for a packet to be sent outside of its host network or network segment. The Internet Protocol (IP) and Internet Packet Exchange (IPX) are both routable protocols.

Non-Routable Protocols

Non-routable protocols cannot survive being routed. In short non-routable protocols have a very narrow world view in that they assume that every computer that they will ever need to communicate with exists on the same network segment that they themselves are on.

Today we find that non-routable protocols are rapidly disappearing from use. The majority of medium to large networks (including the Internet) use routable protocols exclusively. Examples of non-routable protocols include: NetBEUI, DLC, LAT, DRP and MOP.

In Summary

Routing protocols convey routing information such as routing tables, network IDs, hop counts, administrative distance/cost, other metrics and autonomous numbers etc to neighbouring routers. Routable protocols contain data elements such as IP addressing information and are capable of being forwarded from one router to another. Non-routable protocols do none of the above and are restricted to the confines of a single network segment.