Introduction

For connecting a home or an SME to the Internet, we are provided with IP addresses from our ISP. In Europe, RIPE allocate a block of addresses to an ISP and the ISP allows customers to use either a single IP address or a small block. The customer has no permanent rights to the IP addresses, they are part of a provider-aggregable address space or PA space. When you move ISPs, you must change your IP addresses.

Alternatively, you could apply to RIPE for your own provider-independent address space or PI space. You now have a block of IP addresses which are yours. You must find one or more carriers who will agree to advertise your PI space to the Internet. When you move ISPs, you keep your IP addresses. A big block of IP addresses described as a single network (e.g. 193.1.80.0/21) is referred to as a prefix.

In previous notes we have looked at the notion of an autonomous system or AS, a collection of networks acting as a single administrative domain; at the largest scale, the Internet is a collection of such interconnected AS. Large organisations and ISPs will register as an AS and receive a globally unique autonomous system number (ASN). All the PI space IP address prefixes assigned to the organisation are associated with the ASN. This structure allows addresses to be advertised to multiple places on the Internet simultaneously via independent carriers; each network on the Internet can make its way to the AS via the most economical path; this is a multi-homed AS. At the periphery of a network, we might have multiple routers, each connecting to multiple carriers for redundancy. You need to be careful configuring this as you may end up providing transit services across your network otherwise! BGP has filter lists and the ability to use regular expressions to prevent this.

Some company’s primary business is as a carrier providing transit for other AS. There is a pecking-order to carriers and the generally accepted definition is that a tier 1 carrier can connect to every network on the Internet without purchasing transit or paying to peer. A tier 2 carrier is one which purchases at least some of its transit/peering from other carriers.

Most countries and large cities will also have neutral exchange points, where everyone operating in that region can connect economically and at high speed. For Ireland, look up the INEX.

Where two BGP routers connect, there is no hierarchy, they are referred to as peers.

We have covered two other routing protocols in previous lectures (RIP and OSPF). These are interior gateway protocols (IGPs) intended for use inside an AS, they would not scale to something as large as the Internet. To connect AS we need an exterior gateway protocol (EGP) and the protocol currently in use for this purpose is border gateway protocol or BGP. At time of writing BGP-4 is defined by RFC 4271; you should read this for details.

Where we use BGP between AS we refer to this as external BGP or eBGP.

We can also use BGP within an AS, this is internal BGP or iBGP.