IGP and EGP – CompTIA Network+ N10-006 – 1.9

There are many choices for dynamic routing protocols, and there’s a clear line between internal and external protocols. In this video, you’ll learn about IGP, EGP, and the protocols that are used in both of these environments.

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We can generally put dynamic routing protocols into two categories– ones that we use on the inside of our network, and those that we would use on the outside of our network. So in this video, we’ll look at the difference between interior gateway protocols and exterior gateway protocols. Before we can make a determination on what is inside or outside of the network, we need to define an autonomous system, or an AS. An autonomous system means that this is something that is independent of anyone else.

Generally, it is a network that you manage. It is under your control. And as long as this entire network is under your control, it is an autonomous system. If there’s part of the network that is controlled by someone else that would be in a separate autonomous system. If we look at RFC 1930 under Section 3, “An AS is 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 this was probably going to be pretty straightforward. If you start working with routers and you have your own network, there’s always a very clear delineation between what is your network and what is someone else’s network.

As we start discussing the differences between an interior gateway protocol and an exterior gateway protocol, we will absolutely define these based on what the autonomous system might be. And when you start working with routing protocols, the protocols themselves maybe referencing one AS versus another. For all of these ISPs to be able to communicate with each other, they need to be assigned a unique AS, a unique autonomous system number. That ASN is assigned by the Internet Assigned Numbers Authority, or the IANA. This is the same organization that manages the IPv4 address blocks. They manage the root name servers and other administrative parts of the internet.

This is something that you’ll see used if you’re ever taking advantage of a BGP, or Border Gateway Protocol. To be able to route, it needs to be able to know where these autonomous systems are, and it routes from one AS number to another AS number. The number of autonomous systems has grown rapidly from the times of early internet to today. And you can see, it is a straight line up. And because of that, BGP has become a very complex set of protocols that needs some strict administration to be able to work properly. And as long as we have our internet service providers that are working with the IANA, we’re able to route all of our internet traffic successfully. Here’s a very simplified example of how this works.

We have different autonomous systems. These might be different ISPs. And those ISPs have been assigned different AS numbers. We have an AS 100, an AS 200, and an AS 300. And so the routing tables in each one of these routers is set up to know where these different numbers happen to be. So if we need to route information between one internet service provider and another, we’ll simply send it through that network using those BGP routes that take advantage of these autonomous system numbers. If we need to route between autonomous systems, we’re using EGP, or an Exterior Gateway Protocol.

These are the protocols exterior to our autonomous system and allows multiple autonomous systems that may all be configured and managed separately to all somehow now communicate to each other using an EGP. A good example of an EGP is Border Gateway Protocol, or BGP. This is the most popular protocol you’ll find in use, because this is what allows all of the devices connected to the internet to be able to route between each other. It has effectively become the de facto standard for exterior gateway protocols, because of its popularity.

An interior gateway protocol is used on our internal network. It is our own autonomous system. We don’t use IGPs to route between our internal system and someone else’s. If we need to do that, we have exterior gateway protocols that handle that particular function. An example of an IGP might OSPF, Open Shortest Path First. It might be ISIS, intermediate system to intermediate system used by many service providers. There’s the routing information protocol, or RIP. Generally, we see RIPv2 being used today. And we also see EIGRP, which is a Cisco protocol for internal gateways.

This is the Enhanced Interior Gateway Routing Protocol. The interior part is part of the name itself. If you’re managing a rallied network and you have your own dynamic routing, you’re probably working with one of these every day. So if we put all of this together, you would have something that looks like this. We have these multiple autonomous systems at these different companies. Company A is using RIP internally to perform its dynamic routing. Company B is using OSPF. Company C is using EIGRP, and Company D is using RIPv2.

And to be able to communicate across all of them. We have an exterior gateway protocol, a BGP, that’s allowing all of these companies to be able to send routing information, and yet, still be able to run their own routing inside of their networks.