Lecture 16: Internet Routing

An example of the BGP route advertisement process

Now that we know a few algorithms (i.e. LS and DV), we will now take a look at the actual routing algorithms and tools used in the internet. Because neither LS or DV algorithms would converge or be stable in a network the size of the internet, a hierarchical system of routing algorithms is used. Interior Gateway Protocols are algorithms used within autonomous systems, roughly corresponding to organizational networks. Exterior Gateway Protocols are then used to route among the autonomous systems.

Various internal routing algorithms exist, including IS-IS and RIP. On the scale of a single network, either LS or DV sorts of algorithms will work. Cisco also has a proprietary IGP called EIGRP, the Enhanced Interior Gateway Routing Protocol.

The Exterior Gateway Protocol used within the internet is BGP-4, the Border Gateway Protocol, version 4. It is similar to DV algorithms, but the unit of information is a path, so it is called a path vector routing algorithm. We will discuss the information that BGP uses to describe paths, the path attributes. While there are many, many path attributes, we will discuss the commonly used ones, including ORIGIN, AS-PATH, NEXT-HOP, MED and LOCAL-PREF. We will talk about the BGP principles of operation and the route selection algorithm. From this discussion, you should be able to see how an organization controls the routing of packets destined for their networks -- to control peer connections, for instance.

Lesson Objectives

By the end of this lesson, the student will be able to:

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