Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols

IPv6 ND Redirect Function 1 Overview Of Key Routing Protocol Concepts: Architectures, Protocol Types, Algorithms and Metrics

Routing is not just one of the most important activities that takes place at the network layer: it is the function that really defines layer three of the OSI Reference Model. Routing is what enables small local networks to be linked together to form potentially huge internetworks that can span cities, countries or even the entire globe. It is the job done by special devices called routers, which forward datagrams from network to network, allowing any device to send to any other even if the source has no idea where the destination is.

Routing is a complicated subject. The short summary of the process is that routers decide how to forward a datagram based on its destination address, which is compared to information the router keeps in special routing tables. These tables contain entries for each of the networks the router knows about, telling the router which adjacent router the datagram should be sent to in order for it to reach its eventual destination.

As you can imagine, routing tables are critically important to the routing process. It is possible for these tables to be manually maintained by network administrators, but this is tedious, time-consuming and doesn't allow routers to deal with changes or problems in the internetwork. Instead, most modern routers are designed with functionality that lets them share route information with other routers, so they can keep their routing tables up to date automatically. This information exchange is accomplished through the use of routing protocols.

In this section I provide a description of the most common routing (or gateway) protocols used in TCP/IP. I begin with an overview of various concepts that are important to know in order to understand how routing protocols work. I then describe the TCP/IP routing protocols themselves in two subsections. The first covers interior routing protocols, which are used between routers in an autonomous system, and the second looks at exterior routing protocols, used between autonomous systems.

If you don't understand what an autonomous system is or the difference between an interior and exterior protocol, then you know why I included an overview topic on concepts first. In fact, it is really much easier to understand routing protocols once you have a good background on the entire process of routing.

You may notice that in the title of this section I refer to both routing protocols and gateway protocols. These terms are interchangeable, and in fact, the word gateway appears in the name of several of the protocols. This is an artifact of the historical use of the term gateway in early TCP/IP standards to refer to the devices we now call routers. Today, the term gateway normally refers not to a router, but to a different type of network interconnection device, so this can be particularly confusing. The term “routing protocol” is now preferred, and is the one I use.

IPv6 ND Redirect Function 1 Overview Of Key Routing Protocol Concepts: Architectures, Protocol Types, Algorithms and Metrics