Choosing a dynamic routing protocol is an important network design consideration. In this video, you’ll learn about autonomous systems, interior gateway protocols, and exterior gateway protocols.
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Before we get into our discussion of an interior gateway protocol and an exterior gateway protocol, we need to understand what is interior and what is exterior. And to do that, we need to understand an autonomous system or an AS. Autonomous means that something is existing as an independent entity. In this case, we’re thinking about an independent network. It might be a group of IP addresses or IP networks that are under a common control.
If we look at our RFC 1930 section three, it defines an autonomous system as a connected group of one or more IP prefixes run by one or more network operators, which has a single and clearly defined routing policy. And if you are a network administrator at your company, you’re in control of an autonomous system.
The network administrator at the company down the road has their own autonomous system. When we begin our discussion of internal and external gateway protocols, we often refer to autonomous systems because it helps us understand what networks are inside of our control and which might be outside of our control.
An IGP is an interior gateway protocol. It’s a routing protocol that we would use to route between networks that are on our internal autonomous system. We would not use these routing protocols to route to autonomous systems that are outside of our control. We’ll talk about external gateway protocols in just a moment. There are a number of popular internal gateway protocols you might see used with IP version four. One is OSPF version two. It stands for Open Shortest Path First.
There’s also Routing Information Protocol version 2 or RIPv2. And for Cisco networks, you might commonly see EIGRP or Enhanced Interior Gateway Routing Protocol. Of course, there are also routing protocols available for IPv6, things like OSPF version 3, EIGO for IPv6, and RIPng, which stands for routing information protocol next generation.
Of course, our networks go much farther than just our internal communication. We needed some way to communicate to the outside world, to a partner or across the internet. We use an EGP or Exterior Gateway Protocol to provide this communication between autonomous systems. One of the most popular routing protocols used for this exterior gateway communication is BGP or Border Gateway Protocol. If you’re connecting to the internet, you’re probably using BGP as a dynamic routing protocol to that internet connection.
Here’s how all of this might fit together. We have four different autonomous systems. One that’s running the RIP routing protocol. One running EIGRP, another running OSPH, and one running RIPv2. To communicate outside of these autonomous systems, we use the exterior gateway protocol of BGP. This means that we can have an internet connection and all of these devices can route from one side to the other using this BGP protocol.