So now in this series, I'd like to introduce you to the Border Gateway Protocol. We have seen in the introduction so far, we have talked about routing itself, routing protocols, introduced you to Interior Gateway Protocols, and mentioned the Border Gateway Protocol itself. The Border Gateway Protocol is a Routing Protocol used to exchange routing information between different networks. So it's what is known as an exterior gateway protocol. It's described in RFC4271. RFC4271 is the latest version of the BGP implementation specification, and replaces an earlier version written in early 90s RFC1771. Now we're not going to go through the RFC in detail here, because it's very much designed for implementers, software engineers, and so on. However we are going to spend quite a bit of time looking at the BGP specification itself, and how we implement it on the internet today. Now alongside RFC4271, we also have two other documents, supporting documents. RFC4276 is an implementation report on BGP, and it's based on a survey made to various hardware and software developers who have actually implemented BGP on their platforms, and it just documents some of their experiences in dealing with how the standard has been documented, and developing the software and doing the interoperability. We're not going to discuss it here, but if you're interested in some of those experiences, or you're interested in implementing BGP yourself, it's worth having a look. The second RFC, 4277, describes operational experiences with using BGP. And this again is feedback from network operators and other folks who've been using BGP in the years leading up to the publication of this RFC, which was back in 2006. So it has a little bit of history, some of the operational experiences, and some of the things that we're actually seeing on the internet, at least at the time the RFC was being written. Again, a useful bit of background reading if you're interested. Now, as we already mentioned, the Autonomous System is the fundamental part of BGP. BGP operates between autonomous systems. It cares less about the routers, we've seen how the interior gateway protocols allow the routers to talk to each other. BGP is used for the relationship between the autonomous systems themselves, and most specifically it's used to identify networks which have a common routing policy. And this is very much the focus of this presentation series, to show you how to interconnect networks and how to implement routing policies between those networks. BGP is a Path Vector Protocol. I'll show you a couple of slides about what this actually means. It supports or uses incremental updates. Some of the older routing protocols, didn't use incremental updates, what they would do is send their entire routing table every time there was any change in what they were hearing. BGP only announces changes, whether this is a prefix that has disappeared, or whether it's a prefix that has been introduced into the internet. It has a large number of options for policy enforcement, indeed this whole series is about how we implement policies in BGP between autonomous systems. The Internet has been using Classless Inter Domain Routing since 1994. April-May 1994, CIDR was deployed, and of course BGP has been a fundamental part of this CIDR deployment. It's widely used in the Internet backbone, there are over 55,000 different autonomous networks that we can see are using BGP today. And no doubt, there are many others that we cannot see on the Internet, and indeed private networks which are also using BGP. And of course as we've seen, BGP uses autonomous systems. Path Vector Protocol is documented in RFC1322. Again, you can you have a look at that for some offline reading later on. So path vector protocol really defines a route as a pairing between the destination and the attributes of that path to get to that destination. And in that snippet there, I've shown you an example of a destination with the four autonomous systems that the announcement has traversed to get to that destination. And just to show it pictorially, you can see the four autonomous systems mentioned, and the announcement path that these have actually taken. How we get there, which path we follow, which way the traffic goes, of course entirely depends on policies in that network and between those autonomous systems.

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

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