Routers use routing protocols to exchange routing information with each other. There are two types of routing protocols, namely interior gateway protocol, known as IGP, which is used to refer to the process running on routers inside of an ISP's network; and exterior gateway protocol, known as EGP, which is used to refer to the process running between routers bordering directly connected ISP networks. What is an IGP? Interior gateway protocol is used within an autonomous system for carrying information about internal infrastructure prefixes. Two widely used IGPs are OSPF and IS-IS. Why do we need an IGP? We need IGPs for ISP backbone scaling. This is done by helping in providing hierarchy in ISP network. IGPs also limit the scope of network failures. IGPs are only used for ISPs infrastructure addresses not customers or anything else. The design goal is to minimize the number of prefixes in IGP to aid scalability and rapid convergence. What is an EGP? EGP known as the exterior gateway protocol, is used to convey routing information between autonomous systems. It's decoupled from IGPs. The current EGP use is called BGP. Why do we need an EGP? Scaling for light networks. This is done by providing hierarchy and it also limits the scope of network failures. EGP also defines administrative boundaries. It is used for applying policies on a network. For example, it can control reachability of prefixes, it can merge separate organizations, and it can also connect multiple IGPs in the network. So we're going to compare interior versus exterior routing protocols. With IGPs we can do automatic neighbor discovery, whereas with EGPs you need to specifically configure your pairs. For IGPs you generally trust your IGP routers whereas in EGP, you need to connect with outside network so trust is not generally enabled. For IGPs prefixes go to all IGP routers, whereas in EGPs you have to set your administrative boundaries. IGPs usually bind routers in one AS together, whereas in EGPs we're binding AS's together. For IGPs, the IGP is usually responsible for carrying ISP infrastructure addresses only, whereas for EGP, it carries customer prefixes as well as it internet prefixes. IGPs finally, for ISPs aim to keep the IGP small for efficiency and scalability, whereas for EGPs are independent of ISP network topology. If you take a look at the diagram you see that an ISP will typically use IGPs like OSPF or IS-IS to learn infrastructure prefixes within its network. This ISP would use BGP to communicate with other ISPs and IXPs it's connected to. It would either use static routes or BGP to communicate with its customers. These prefixes it learns from its customers, IXP, or other ISPs, are typically propagated within its own network using BGP. We will look at how this is done in the next series.

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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