The Illustrated Network- P42:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER Border Gateway Protocol 15 What You Will Learn In this chapter you will learn about the BGP and the essential role it plays on the Internet. With BGP routing information is circulated outside the AS and to all routing domains. We ll see how a simple routing policy change can make a destination unreachable. You will learn about the differences between the Internet BGP IBGP and the Exterior Gateway Protocol EBGP and why both are needed. We ll also look at BGP attributes and message formats. The EGP used on the Internet is the Border Gateway Protocol BGP . IGPs run between the routers inside a routing domain single AS . BGP runs between different autonomous services ASs . BGP runs on links between the border routers of these routing domains and shares information about the routes within the AS or learned by the AS with the AS on the other side of the border. BGP makes sure that every network and interface in any AS located anywhere on the Internet is reachable from every other place. BGP does not generate any routing information on its own unlike the IGPs which essentially bootstrap themselves into existence. BGP relies on an underlying IGP or static routes as the source of the BGP-distributed information. BGP runs on the border routers of Ace ISP s AS 65459 routers P9 and P4 and Best ISP s AS 65127 routers P7 and P2 . These are highlighted in Figure . An IGP such as OSPF or IS IS runs on the direct links between routers P9 and P4 and routers P7 and P2 but these are interior links. BGP runs on the other links between the backbone routers. BGP AS A ROUTING PROTOCOL There are EGPs defined other than BGP. The Inter-Domain Routing Protocol IDRP from ISO is the EGP that was to be used with IS IS as an IGP IDRP is also sometimes promoted as the successor to BGP or the best way to carry IPv6 routing information 380 PART III Routing and Routing Protocols bsdclient Inxserver winclil winsvrl Ethernet LAN Switch with Twisted-Pair Wiring the last two .
